Would it say it on the work order from my last oil change at the dealer?
It might. If not, it would seem logical that they would only put Mobil One in the N* V8 unless you told them to use something else. If nothing else, the price for a Mobil One change will be anywhere from $45 to $85 dependent on the area and your dealer.
Whatever oil you use, just remember to check it. I usually have to add a quart to my 3.6 when the OLM drops below 50%. I loaned the car to my sister and brother-in-law for three weeks. When I checked the oil this weekend, the OLM was at 44% and I needed to add just under a quart.
Use only GM approved synthetic. There are only a few. I think I remember the manual said if anything else is used, remove ASAP. Use anything else you may have a warranty problem if the dealer knows. I also have a vague recollection that someone put the wrong oil---was not synthetic or the wrong type synthetic--in their engine and had to undergo several very quick oil changes in succession to get it out.