Stick with the Mobil 1 it's great oil why would you use a dino in that engine when it calls for synthetic?? If your engine uses oil later it won't have anything to do with using a quality Mobil 1 oil..That is whats in those engines when they come from the factory, if it was a problem GM would not use it..The old way of thinking when synthetic was a "real" synthetic and engines were not built so well has nothing to do with today's engines and oils.
Whenever we rebuilt engines requiring synthetic oil we always filled it with synthetic. Period!