Put the CTS up on the jack and jack stands and re-tightened one cat-back pipe connecting sleeve. I have one sleeve that comes loose every several months; the symptom
is that the exhaust gets a slightly higher tone. I need to have it welded, or perhaps replace this sleeve, or add in some locktite.

Dropped by Walmart and picked up 6L of Mobil 1 5W30. Had to be careful to select between normal Mobil1, high mileage, Truck/SUV, and high-performance (or was it high endurance?).
The main difference among these appears to be that anything other than the regular oil is 20% more expensive.
The 5L container was $19, and the 1L extra I need for the CTS was $5.67. Hmm. I wish manufacturers & oil companies would settle on how many liters are needed for an oil change
and just package for that.

The CTS is an easy oil change. Two bolts, no real need to even put it up on ramps; if you turn the wheels to the side you can reach in behind the passenger wheel to
access the oil drain plug. The drain is near the catalytic converter on that side, so best to wait for that to cool off.

I should mention that my mechanic attempts either tend to go swimmingly well, or become an adventure. On the bright side, sometimes this is how we learn.
Other times it just becomes amusing.

First, I found that the oil drain plug was on more tightly than I could twist a wrench reaching in from the side. So, I put the car up on the jackstands, allowing me to slide under
and get a better angle on the problem. Next, I discovered that I was not really using the correct socket, so I went off to rummage through and find my 13mm socket.
The 13mm socket once I found it I discovered is 'popped', or cracked. Luckily I dug out a new tool set and grabbed the 13mm socket out of that.
Once I got back under the car and arranged the oil catch pan I popped the new socket as well. Yes, the oil drain plug is on snugly alright.

I considered wrenching it, but decided the wise course was to put the CTS off the stands, drive to PepBoys, get a new socket, return home, and wait for the cats to cool.
Picked up lunch on the way back.

Wife ready to be taken car shopping. Asked for a 20 min reprieve to finish my 5 min oil change.

I got two 13mm sockets at the store; one shorter, with a star-grip pattern, and one longer with a hex pattern. I figure this gives me at least 2 chances to get the
oil drain plug off. Put the car back up on the jack/jackstand. The oil drain plug comes right off, no drama. The oil does look good and dirty, and almost all
of it gets into the catch pan.

The CTS has a permanent oil filter enclosure, which takes a cannister type filter element. Easy access on the top of the engine to the filter enclosure,
and it has a single large bolt on top. My largest crescent wrench opens enough to fit this bolt (score!). However, there is not a single good angle
that does not have a sensor in the route of twisting of the wrench. By this I recall from previous errors that if there is a sensor in the path of the wrench,
if the wrench manages to come off the bolt being loosened, the wrench will then dramatically remove the sensor from the car resulting in a longer and more
expensive repair.

As a kludge I attempt to put the wrench straight down on the bolt, then use a screw driver through the hole in base of the wrench to form a sort of T
and apply a twisting motion. This is successful in opening the filter enclosure. No innocent sensors are stripped from their moorings.

The filter enclosure cover is hotter than my tolerance for holding hot things I discover, but I manage to keep it in hand over to get a paper towel
under it, change the filter element, and get it back screwed down.

Cleaned and replaced the drain plug, now down to the last drops. Used a funnel to pour in the 6L's of fresh oil. Then poured
the used oil from the catch container back into the Mobil 1 bottle for return to the oil recycling at walmart. The Mobil 1 bottle tends
to be cleaner for the trip back to the store in the Cadillac.

Synthetic Oil change: $26 in oil, $10 filter, one minor injury while wrenching which looks worse than it is, $10 in new sockets: $46.
The satisfaction of knowing that it was done properly with the appropriate oil: priceless.

Dropped the oil off on the way to take my Wife car shopping. We looked at a Toyota Camry Hybrid, and a Prius Hybrid. We wanted to
look at some Psions but never got a salesperson to help with those so we just left. The Prius has more interior room than I expected.