I don't have a clue what oil my GM dealer puts in. Don't really think they would put something in that would void waranty.
I just change it every 3K at dealer. I find it cheaper than most places and even cheaper than I can buy oil and filter for.
My dealer has a pay for 3 and get one free. Bottom line that works out to about $22 an oil change. Not to mention they look at about 30 other things.
IMHO, I don't think there is much difference from one brand to another. I even tried using Rural King oil, may be citco, in a tractor. It lasted as long as the brand names, 75 hours without using a drop.
Oil brands may be just like tractor brands, is red better than blue, green?