When Should I Start Using High Mileage Oil

[JohnnyG]

JohnnyG,

I am sure you already know this, but just in case...

Your Cadillac V8 qualifies as a ULEV...

Ultra Low Emissions Vehicle...just like some of the hybrids and super economy cars.

Actually Jim, I didn't know that since I didn't get a fancy Japanese style sticker in my rear quarter window, but it doesn't surprise me. I'm sure GM just couldn't afford the sticker!

And I'm still PO'd every time I see the new Lexus (or is it Infinity?) commercial stating that they are the first to offer MR shock fluid in their cars! The 2004 STS had that. GM never did know how to capitalize on their accomplishments did they?

Anyway, I still firmly believe that pre 2000 Northstar oil consumption, and yes to some lesser degree after the 2000 engine redesign, is largely due to lubrication failure. The new GF-5 standard appears, to me at least, to address those failures.

[Texas Jim]

JohnnyG,

I am sure you already know this, but just in case...

Your Cadillac V8 qualifies as a ULEV...

Ultra Low Emissions Vehicle...just like some of the hybrids and super economy cars.

Actually Jim, I didn't know that since I didn't get a fancy Japanese style sticker in my rear quarter window, but it doesn't surprise me. I'm sure GM just couldn't afford the sticker!

And I'm still PO'd every time I see the new Lexus (or is it Infinity?) commercial stating that they are the first to offer MR shock fluid in their cars! The 2004 STS had that. GM never did know how to capitalize on their accomplishments did they?

Anyway, I still firmly believe that pre 2000 Northstar oil consumption, and yes to some lesser degree after the 2000 engine redesign, is largely due to lubrication failure. The new GF-5 standard appears, to me at least, to address those failures.

About the MR...I agree with you.

It irritates me too..

My car also has MR..I love it.

[Rich]

I'm not sure I understand JohhnyG's comment regarding oil consumption on pre 200 Northstar motors being the result of "lubrication failure". My 1997 Northstar does have a consumption issue, especially when driven at higher highway speeds. Exactly what type of lubrication failure do yoy mean?

[JohnnyG]

I'm not sure I understand JohhnyG's comment regarding oil consumption on pre 200 Northstar motors being the result of "lubrication failure". My 1997 Northstar does have a consumption issue, especially when driven at higher highway speeds. Exactly what type of lubrication failure do yoy mean?

Rich, the kind of lubrication failure that I'm talking about has nothing to do with bearing wear, or camshaft wear, even in flat tappet engines. What I'm talking about is lubrication failure under the most demanding high temperature areas in our Northstar engines. That would be the ring pack and upper part of the piston. This part of the Northstar is specifically designed to reduce emissions by eliminating almost all of the space between the flame front (combustion chamber). This reduces the amount of "hidden" fuel mixture, which would be left uinburned and sent out the tailpipe. It also has some wonderful side benefits, like higher fuel economy and more power per cylinder. Motor oil additives of the time were just not up to the task.

I call it a failure in lubrication because if there wasn't, we would not have ring carbonization or stuck rings, we would not have the (now gone) ring cleaning procedure (GM Service) for the Northstar. And we would not have excessive oil consumption. I know that the Guru once blamed high oil consumption on the chance meeting of worn tools and resulting excessive tolerances in some engines, but I never really bought into that. A ring cleaning process cannot fix tolerances, and we all can't go out and do a WOT every so often, so to me it was a smokescreen for oil issues.

Specifically, from all that I've read over the years, VII's used in motor oil was (is) the main culprit. To which now I am advocating the new GF-5 standard specification of motor oil which specifically mandates MINIMAL ring deposits. There's a link hidden in my rants somewhere in this series of posts.

Thanks for asking.

[Jan Olsson]

The late 2002 STS had MR as an option. I have it on my car.

If you have oil consumption then either you don't do enough WOTs or, as the guru explained, that particular engine uses more oil. I have never had a problem with oil consumption on a Northstar for instance.

What do you mean with failure? I would say that they certainly do get lubricated, just as intended. Read up

You sound like every other Northstar has carbon build ups etc. Nothing is wrong with using the latest oils as long as they have the right viscosity and as long as one remembers that the API-grades usually but not always (ZDDP) superseeds the earlier grades.

As an engineer I'm very familiar with production tolerances. Tools get worn and replaced all the time, thus making the tolerances vary (within specs of course). I can't see anything wrong in what the guru says.

[JohnnyG]

What do you mean with failure? I would say that they certainly do get lubricated, just as intended. Read up

Yes, they get lubricated as intended, no excessive wear as I said in my reply to Rich. The VII's coked up and froze the ring packs in many cases, and NO, not all have excessive oil usage. In fact the original design as I recall was for the engine to use some oil in order to lubricate the rings, some people deemed any usage to be excessive in an expensive car. The post 2000 engine redesign moved the top ring down away from the combustion chamber to reduce the heat stress. It also incorporated higher tension rings for the same reason. Horsepower was reduced slightly in the process.

You sound like every other Northstar has carbon build ups etc. Nothing is wrong with using the latest oils as long as they have the right viscosity and as long as one remembers that the API-grades usually but not always (ZDDP) superseeds the earlier grades. (?)

As an engineer I'm very familiar with production tolerances. Tools get worn and replaced all the time, thus making the tolerances vary (within specs of course). I can't see anything wrong in what the guru says.

His point was that new cylinder hones gave the early blocks for that tool run too much crosshatch, thereby increasing oil comsumption. My point is, oil additives took over from that point (along with overfilling the engine with oil and a few other little details) and increased oil consumption to the point where it exceeded the (generous) 1 quart/1000 mile limit. This forced GM to develop the ring cleaning process, along with customer complaints of course.

You see, I have read up.

[Jan Olsson]

Well ok, you have read up, sorry.

But I still insist that the WOT-treatment is the best cure. Certainly one should use the newest available oil that suits the vehicle.

I was just trying to make clear that the newest available oils aren't always a good idea.

His point was that new cylinder hones gave the early blocks for that tool run too much crosshatch, thereby increasing oil comsumption.

Not too much, rather more crosshatch than a late production run block.

It is a well known fact that cars driven very "carefully" tend to get problems with sticking valves, deposits etc. I would say that the "problems" still wouldn't exist if the drivers had driven the cars a bit harder. The new oils will help preventing the problem but the root cause is still driver inflicted "damage" in the most cases with high-performance engines (i.e. engines designed to be run hard).

Keep in mind the difference between design intentions and reality. The slowest running cars around here always seem to be the cars with the highest power outputs.

The guru once said "People are screaming for higher power and when they get higher power they don't use it!"

I can't say that there is a problem in the design if it works as it should, but when people don't use it as they should, then there is a problem.

[Texas Jim]

Well ok, you have read up, sorry.

But I still insist that the WOT-treatment is the best cure. Certainly one should use the newest available oil that suits the vehicle.

I was just trying to make clear that the newest available oils aren't always a good idea.

His point was that new cylinder hones gave the early blocks for that tool run too much crosshatch, thereby increasing oil comsumption.

Not too much, rather more crosshatch than a late production run block.

It is a well known fact that cars driven very "carefully" tend to get problems with sticking valves, deposits etc. I would say that the "problems" still wouldn't exist if the drivers had driven the cars a bit harder. The new oils will help preventing the problem but the root cause is still driver inflicted "damage" in the most cases with high-performance engines (i.e. engines designed to be run hard).

Keep in mind the difference between design intentions and reality. The slowest running cars around here always seem to be the cars with the highest power outputs.

The guru once said "People are screaming for higher power and when they get higher power they don't use it!"

I can't say that there is a problem in the design if it works as it should, but when people don't use it as they should, then there is a problem.

That's a statement that I can agree with...

[CarlaValentine]

Well ok, you have read up, sorry.

But I still insist that the WOT-treatment is the best cure. Certainly one should use the newest available oil that suits the vehicle.

I was just trying to make clear that the newest available oils aren't always a good idea.

His point was that new cylinder hones gave the early blocks for that tool run too much crosshatch, thereby increasing oil comsumption.

Not too much, rather more crosshatch than a late production run block.

It is a well known fact that cars driven very "carefully" tend to get problems with sticking valves, deposits etc. I would say that the "problems" still wouldn't exist if the drivers had driven the cars a bit harder. The new oils will help preventing the problem but the root cause is still driver inflicted "damage" in the most cases with high-performance engines (i.e. engines designed to be run hard).

Keep in mind the difference between design intentions and reality. The slowest running cars around here always seem to be the cars with the highest power outputs.

The guru once said "People are screaming for higher power and when they get higher power they don't use it!"

I can't say that there is a problem in the design if it works as it should, but when people don't use it as they should, then there is a problem.

That's a statement that I can agree with...

I second that Jim.

My Northstar sees well over 500 miles a week, about 90% of which is highway, and you better believe that gas pedal gets a little "extra shove" when getting on the highway. I have noticed that the more I "use" it, the better it seems to run. It has more get-up-and-go and less oil consumption than it did before. I also NEVER see dark smoke from my exhaust as I did the first couple months after I got it. I believe the prior owner babied it a bit.

[Rich]

Thanks all for your replies. I just want to be clear regarding my oil consumption. My 97 Eldorado ETC now has 124,000 miles. I purchased the car new and have always maintained the car better than what is called for in the manual. I have never "babied" the car and have always driven her as she was designed to be driven. The WOT procedure is a regular part of my routine. The oil consumption has seemed to increase over the past two years or so as my driving cycle has changed somewhat. I drive approximately 1,000 per month, mostly on the expressway at 75 to 85 MPH. I've been using Mobil 1 10W-30 High Mileage oil for years now, but over the past two years the motor has consistenly consumed approximately 1 1/2 (One and one-half) quarts of oil per 1,000 miles driven. Before adopting this driving cycle, the Eldorado used somewhat less oil, but she did require the occasional bit of oil between changes. Not sure if there is a problem here as I've read much if not all of the oil posts on this forum including those by the Guru. The car runs great, but she does use some oil. For comparison, my wife's 2006 Northstar STS uses absolutely no oil between changes. I'm not saying anything is wrong, that's just how it is with my two Cadillacs. I have no complaints. These cars are both great and the Eldorado has become a personal friend of mine.

[Texas Jim]

Thanks all for your replies. I just want to be clear regarding my oil consumption. My 97 Eldorado ETC now has 124,000 miles. I purchased the car new and have always maintained the car better than what is called for in the manual. I have never "babied" the car and have always driven her as she was designed to be driven. The WOT procedure is a regular part of my routine. The oil consumption has seemed to increase over the past two years or so as my driving cycle has changed somewhat. I drive approximately 1,000 per month, mostly on the expressway at 75 to 85 MPH. I've been using Mobil 1 10W-30 High Mileage oil for years now, but over the past two years the motor has consistenly consumed approximately 1 1/2 (One and one-half) quarts of oil per 1,000 miles driven. Before adopting this driving cycle, the Eldorado used somewhat less oil, but she did require the occasional bit of oil between changes. Not sure if there is a problem here as I've read much if not all of the oil posts on this forum including those by the Guru. The car runs great, but she does use some oil. For comparison, my wife's 2006 Northstar STS uses absolutely no oil between changes. I'm not saying anything is wrong, that's just how it is with my two Cadillacs. I have no complaints. These cars are both great and the Eldorado has become a personal friend of mine.

My 2006 DTS also uses no oil between changes.

It is usually changed at around 35% left on the oil monitor..which is about 5000 to 7500 miles on the oil.

[Cadillac Jim]

I have a 1997 ETC that I bought new. After about 80,000 miles I started having problems that looked like dino oil instead of the Mobil 1 I was told was being used. After never having had to add oil, ever, it went to burning a quart every 500 miles in one oil change. The GM treatment for stuck rings says that you leave the solvent in the cylinders for at most one hour, or the shellac will settle back down into the rings and the car will be as bad or worse than before. I had it done and the tech let it settle for 4 1/2 hours. I struggled with this for awhile, then changed where I had the car serviced. Two oil changes and a highway trip, and oil consumption went down to a quart every 2,000 miles.

Change where you get your car serviced. Consider using Mobil 1, at least for just one oil change. After you get what you *know* is good, fresh oil in the engine, either take a 500-mile trip or do some WOTs. Doing WOTs or highway driving with bad oil doesn't help.

[JohnnyG]

My 2006 DTS also uses no oil between changes.

It is usually changed at around 35% left on the oil monitor..which is about 5000 to 7500 miles on the oil.

Jim, I just want to remind you that the 2006 and the 1997 are different engines due to the redesign of the pistons and rings. In fact I think your 2006 required synthetic oil right from the start. That, combined with your spirited driving undoubtedly has kept your oil usage at zero. I think overall that Jims_97 understands what I am trying to say here. It's no great feat to keep a 2000+ engine's oil consumption at a minimum, but pre-2000 is another story altogether.

And back to Rich for a second here. His statements seem to confirm exactly what I am saying, except I don't know what his

I've been using Mobil 1 10W-30 High Mileage oil for years now

statement means. It's just too vague to be considered as proof that M1 is better, not to mention the fact that I am not a fan of High Mileage anything as I said at the start of this discussion.

His results however seem to support the use of synthetic oil (and the WOT) in pre-2000 engines as a means to keep oil consumption low.

[Rich]

Sorry for taking so long to reply. I've been using Mobil 1 10W-30 High Mileage oil for several years now but I really don't know if reduces oil consumption. As I said above, my 1997 Eldorado (124,000 miles) does consume 1 1/2 (One and one-half) quarts of oil per 1,000 miles driven on my current driving cycle, which consists of mostly highway driving at 75 to 85 MPH. Seems a bit much to me especially considering the oil I'm using, which is formulated in part to reduce oil consumption. But the car performs brilliantly, and I've read that these motors tend to use oil by design, so it is what it is. The other reason I use high mileage oil is that the formulation supposedly contains a higher amount of seal conditioners than other oils in order to minimize leaks. To this I can attest that my Eldorado doesn't leak so much as a drop of oil.

[JohnnyG]

Rich, I stopped by WallyWorld today and checked out the oil isle. I found out that Mobil 1 High Mileage is only rated as an API SL oil, even though it states that it is fully synthetic. Though I'm not sure exactly which parameters of SM classification that it fails, I'd bet that it has higher ZDDP levels than SM calls for. This probably is a good thing for your 97 engine.

All I can say is if it's working for you, then keep on using it.

[Rich]

JohnnyG,

Yes, I should have mentined that too. The API certification is SL CF, and it qualifies as ILSAC GF3, not as ILSAC GF4. I understand this is beneficial relating to ZDDP levels.

Rich

[Jan Olsson]

Rich, I stopped by WallyWorld today and checked out the oil isle. I found out that Mobil 1 High Mileage is only rated as an API SL oil, even though it states that it is fully synthetic. Though I'm not sure exactly which parameters of SM classification that it fails, I'd bet that it has higher ZDDP levels than SM calls for. This probably is a good thing for your 97 engine.

All I can say is if it's working for you, then keep on using it.

The API-grades has nothing to do with if the oil is synthetic or not. If you read my previous post quoting the guru earlier on then you would also have seen that oils that classifies for both gasoline and diesel engines also has a higher anti-wear protection. Diesels still need it.

I would recommend the Rotella/Delvac/Delo oils for the 93-99 Northstars, personally. The latest ILSAC fuel economy oils are getting very low in ZDP concentration. Fine with the newest engines on the road but not so fine for a 93 Northstar.

You can get the Rotella/Delvac/Delo oils in 10W30 for your engine but they are much harder to find. Check a truck stop or large equipment supplier. If all you can find is the 15W40 version of the Rotella/Delvac/Delo that is fine in the 93 Northstar. The oil is not that much thicker to hurt anything unless you live in the far north and need to cold start at -40.

The other option is to use one of the "severe service" oils marketed by Mobil that are designed for longer change intervals and marketed as such. If you read those bottles you will notice that they also do NOT meet the ILSAC "for gasoline engine" use as they do not have the ILSAC starburst symbol. That is because those oils designed specifically for longer change intervals also have more ZDP in them that excedes the amount allowed for the ILSAC starburst designation.

[Cadillac Jim]

JohnnyG -- Of course I understand that you are saying that the later Northstars don't have the problem of the rings sticking with prolonged sedate in-town driving, as apparently the 1993-1999 Northstars did. I have no knowledge of that myself, having a lead foot, straight-through mufflers, and a liking for Interstate cruising -- note my tag line "Touring is it's middle name" and the fact that I have always used synthetic, mostly 5W-30, from day 1 with the new car in 1997. But it did develop a really severe oil-burning problem due to what was apparently poor maintenance at about 80,000 miles, that solved by changing the maintenance.

[mike27513]

Rich, I stopped by WallyWorld today and checked out the oil isle. I found out that Mobil 1 High Mileage is only rated as an API SL oil, even though it states that it is fully synthetic. Though I'm not sure exactly which parameters of SM classification that it fails, I'd bet that it has higher ZDDP levels than SM calls for. This probably is a good thing for your 97 engine.

All I can say is if it's working for you, then keep on using it.

The API-grades has nothing to do with if the oil is synthetic or not. If you read my previous post quoting the guru earlier on then you would also have seen that oils that classifies for both gasoline and diesel engines also has a higher anti-wear protection. Diesels still need it.

I would recommend the Rotella/Delvac/Delo oils for the 93-99 Northstars, personally. The latest ILSAC fuel economy oils are getting very low in ZDP concentration. Fine with the newest engines on the road but not so fine for a 93 Northstar.

You can get the Rotella/Delvac/Delo oils in 10W30 for your engine but they are much harder to find. Check a truck stop or large equipment supplier. If all you can find is the 15W40 version of the Rotella/Delvac/Delo that is fine in the 93 Northstar. The oil is not that much thicker to hurt anything unless you live in the far north and need to cold start at -40.

The other option is to use one of the "severe service" oils marketed by Mobil that are designed for longer change intervals and marketed as such. If you read those bottles you will notice that they also do NOT meet the ILSAC "for gasoline engine" use as they do not have the ILSAC starburst symbol. That is because those oils designed specifically for longer change intervals also have more ZDP in them that excedes the amount allowed for the ILSAC starburst designation.

In the US, Shell distributors have Rotella 5W-40 synthetic, what is your opinion of running this in a Northstar? Any concerns?

[Ranger]

Your engine was spec'd for a 5W30. Why do you want to go to a 5W40?

[mike27513]

Your engine was spec'd for a 5W30. Why do you want to go to a 5W40?

Well, from my reading at various sites [bob is the Oil Guy is a good site with detailed analysis] the advantages of synthetic 5W-40 oil show up in extended High RPM, High Performance use- for example engine oil at 6000 RPM+ will increase in temperature to over 300+ F as the oil on friction surfaces flows between surfaces. Synthetic 5W-40 will not thin out or break down as easily as 5W-30 Synthetic or Dino oil. [some High-Po cars have additional oil coolers to deal with this.]

The 5W-40 has been used in some Foreign makes and I was curious if it had been used by Northstar users here.

My particular Application is my wife's XLR, which my Knucklehead son likes to wind up occasionally to 6000+ RPM.

Also this Rotella 5W-40 since it is for Diesels has more wear inhibitors than standard automotive oils.

Just curious to see if anyone has experience with these and the possible downsides.

[Ranger]

A couple thoughts come to mind.

1. How long does your engine run at 6000 RPM?

2. An occasional dose of WOT is beneficial to the Northstar and recommended.

3. When the Northstar was being tested it was run at WOT for 300 hours (12.5 days) straight. Now remember, it was spec'd for conventional 10W30 so that's what it was tested on. That should dispel any worries about it thinning out or breaking down.

4. I seriously doubt that the oil ever gets to 300 degrees. Conventional oil will not survive at those temps. That is why you get a change oil message if you overheat the engine and that comes on at about 260 or 265. The engine has an oil cooler which passes engine oil through a heat exchanger in the radiator side tank.

5. While Bob is the oil guy is a good site with lots of info, keep in mind two things. 1. Those guys are fanatics. They have oil in their veins and get a transfusion every three months. 2. I believe that site is sponsored by oil additive manufacturers.

Even though mine was spec'd for 5W30 I still use 10W30. Either one should serve you well.

[Texas Jim]

Your engine was spec'd for a 5W30. Why do you want to go to a 5W40?

Well, from my reading at various sites [bob is the Oil Guy is a good site with detailed analysis] the advantages of synthetic 5W-40 oil show up in extended High RPM, High Performance use- for example engine oil at 6000 RPM+ will increase in temperature to over 300+ F as the oil on friction surfaces flows between surfaces. Synthetic 5W-40 will not thin out or break down as easily as 5W-30 Synthetic or Dino oil. [some High-Po cars have additional oil coolers to deal with this.]

The 5W-40 has been used in some Foreign makes and I was curious if it had been used by Northstar users here.

My particular Application is my wife's XLR, which my Knucklehead son likes to wind up occasionally to 6000+ RPM.

Also this Rotella 5W-40 since it is for Diesels has more wear inhibitors than standard automotive oils.

Just curious to see if anyone has experience with these and the possible downsides.

As Ranger noted...Your car "DOES" have an oil cooler.

As for the occasional high RPM...

My 2006 DTS Performance Sedan, now has 91,000 miles on it.

It has seem occasional high RPM's since day 1.

My rev limiter is set at 7000 RPM...not 6000.

At least once a week...I STAND ON IT and let it wind up thru 1st and 2nd.

On long trips...sometimes it is run for hours between 3500 and 4000 RPM

It hasn't hurt anything yet...and it uses absolutely -0- oil between changes.

It also seems just a tiny bit peppier with Synthetic oil than with DINO oil..,but that may just be in my head.

[mike27513]

Your engine was spec'd for a 5W30. Why do you want to go to a 5W40?

Well, from my reading at various sites [bob is the Oil Guy is a good site with detailed analysis] the advantages of synthetic 5W-40 oil show up in extended High RPM, High Performance use- for example engine oil at 6000 RPM+ will increase in temperature to over 300+ F as the oil on friction surfaces flows between surfaces. Synthetic 5W-40 will not thin out or break down as easily as 5W-30 Synthetic or Dino oil. [some High-Po cars have additional oil coolers to deal with this.]

The 5W-40 has been used in some Foreign makes and I was curious if it had been used by Northstar users here.

My particular Application is my wife's XLR, which my Knucklehead son likes to wind up occasionally to 6000+ RPM.

Also this Rotella 5W-40 since it is for Diesels has more wear inhibitors than standard automotive oils.

Just curious to see if anyone has experience with these and the possible downsides.

As Ranger noted...Your car "DOES" have an oil cooler.

As for the occasional high RPM...

My 2006 DTS Performance Sedan, now has 91,000 miles on it.

It has seem occasional high RPM's since day 1.

My rev limiter is set at 7000 RPM...not 6000.

At least once a week...I STAND ON IT and let it wind up thru 1st and 2nd.

On long trips...sometimes it is run for hours between 3500 and 4000 RPM

It hasn't hurt anything yet...and it uses absolutely -0- oil between changes.

It also seems just a tiny bit peppier with Synthetic oil than with DINO oil..,but that may just be in my head.

Yes, All of which I agree- My comments specifically were that some Euro cars like Ferrari's / Lambo's which turn 8-10,000 RPM spec Synthetic 5W-40. Also some VW/ Audi Turbo Diesels.

Just looking for the "ideal" oil and feedback experiences from actual Northstar users [especially European, I hope].

The Synthetic Rotella 5W-40W at $14/ gallon is more cost effective than the Mobil 1 Synthetic at $22 gallon. [2 gallons for the XLR]

And hopefully some Chemical /Petroleum Engineer & Northstar owner can make some recommendations

I run Mobil 1 Synthetic 5W-30 which is the GM recommended XLR fill and Synthetic [or less frequently Conventional] 5W-30 in my Deville.

My 5.4L Expedition runs Ford Synth 5W-20 which is spec'd and even a 10,000 lb towing load hasn't so far hurt anything. It is still under the 6 yr/60K Powertrain warranty so I use the Ford required parts/fluids. Son's Malibu and Daughter G6's get 5W-30 Synth.

Your experience with Synthetic "Peppy" is probably not in your head depending on the type you run.

Tests show Cold Engine Start RPM is higher since most Synthetics flow better at cold than most Dino oils cold.

And most synthetics seem to result in the engine with less restriction/friction & better flow rates.

The newer Synthetic 0W-30 oils actually test out with slightly higher fuel efficiency than 5w-30s with equal protection.

I will be experimenting with these this winter. [Most of my ramblings are taken from test results from the "Bob" oil site]

As with anything, People are free to use what they want to use. Just wanting some actual experiences from N* users and maybe some pointers to sites with test data.

[JohnnyG]

Jan, I understand what you mean about the oil ratings, I only put that comment in my post as a way to say that it didn't seem to make sense economically to have a synthetic oil rated as SL these days. In fact I have some Mobil1 15W-40 from the early 80's that is rated SF! Sorry if I misled you by including that, I was going to edit it out before posting but didn't.

In the US, Shell distributors have Rotella 5W-40 synthetic, what is your opinion of running this in a Northstar? Any concerns?

Yes Mike27513, they are all over that Rotella and diesel oil in general over at Bob's the oil guy aren't they. I've also heard a lot of recomendations for the Northstar to use diesel oil, but it wouldn't be my choice. Nobody ever specifies the weight ratings when they are making those recomendations and in my opinion, if you could find a 10W-30 diesel oil, or even a straight SAE30, you'd be better off. The oil would likely meet SL rating much the same as the High Milage oil mentioned above however, so I'm not sure it would make sense. I'd definitely not use a 15W-40 diesel oil in an older Northstar, but I've known people that have with no ill effects. In the end, it's more about fuel economy at low temps than anything else, straight weight oils and 15W-40 won't give you that, or cold temp pumpability (GM 4718M or 6094M?).

Your choice of 5W-40 Rotella might be OK, but I'd still look for it to meet the GM specs before I used it. I don't think it will. As I suggested to Texas Jim at the start of this thread, if I were looking to improve my engine oil, I'd be looking for a GF5/SN these days. They will handle the coming ethanol overload much better and give you that protection at least.

Come to think of it, that another reason not to use diesel oil. There is no ethanol in diesel fuel, nor consideration for a lot of short trip driving, so acid/moisture protection is pretty much non existent with it.