Just a point about synthetic oils. There are good/better/best. The best is Group 4 PAO oils (polyalphaolephin), which are Mobil 1, Amsoil, Redline, and Royal Purple. Everything else that calls itself synthetic, even "full synthetic" is a Group 3 oil, which is super refined dino oil. Now, Group 3 oils are far better than the usual Group 2 "dino" oils. If GM's OLM is programmed with Group 2 oils, use it and you'll achieve the life GM has designed into its engines. If you want the best possible mileage/lowest wear/lowest cost ratio, use a PAO oil and used oil analysis.
It seems to me a nice compromise for those non oil-enthusiasts, is to use a Group 3 oil (Castrol, Quaker State, Penzoil synthetics, etc) and change it when the OLM says. This way you're changing the oil "early" so maximizing engine protection, you're keeping up with the GM warranty so no hassles if anything goes awry, and you're changing oil less often which is less hassle, and less used oil to dispose of so more kind to the environment.
My compromise is to use Amsoil Series 2000 Ow30, and change it once a year. I put their new nano-fibre oil filter in, which also only needs to be changed once a year. The oil is good for 35,000 miles normal service or 17,500 mile severe service, the oil filter for 25,000 miles or at oil change. Amsoil defines severe service as:
Turbo/supercharged vehicles, commercial or fleet vehicles, extensive engine idling, first and subsequent use of AMSOIL in vehicles with over 100,000 miles, daily short trip driving less than 10 miles (16 km), frequent towing, plowing, hauling or dusty condition driving.
I do none of those, so my recommended OCI is once a year, oil and filter. I like it.
