Oil is what I recommend, at the end of the day, I have never heard of a Duramax that blew up since someone put the wrong brand 15w40 or 5w40 in it. It comes down to how you use your vehicle. You can put the best or worse oil, change it every 3-5 thousand miles in DD application and be fine. If it's a truck thats beat hard, towing or runs hot, then synthetic on normal change rotation.Buddy. Lol.
your going to get answers all over on this one.