They can, and do, require whatever oil you use to meet said specs for warranty coverage.
I haven't seen any owners manual that says "Use whatever oil and or weight you want."
BINGO, I thought I was the only person who realized that! Thanks Man. Bottom line pick an approved oil from GM's list if you own a GM. If you have a problem and follow GM's recommendations you'll be fine. Even cheap no-name oils are on the list, so price and availability is no problem!