Amsoil does not sell to any manufactures, so GM did not install it. Dealer sell it though. There are 2 GM dealer I am aware of the have it in the parts department along with Mobil 1. Customer gets what they want.
That said, I have heard of some Corvette specialty dealers who do this. They install Amsoil products, oil, lubes, filters etc. They also will install other products but these are not the normal Chevy dealers.
btw-The rumor was floating around the net they were making the new Harley-Davidson synthetic oil too but that was just rumors and not true. Amsoil claims they would not sell there products to be repackages under another brand name or with recognition.