All GM dealers that buy from GM"s dealer oil program have been on "synthetic" AC Delco Dexos 1 Gen 2 and gen 1for at a minimum of 5 years now. They only offer blend on 5w30 and 5w20 which are not Dexos spec oils. So its a 95% chance they put 0w20 synthetic.
The AFM transition thing makes no sense blend vs. synthetic. I can see changing oil weight having some sort of change there as a 30 weight oil is of a thicker viscosity than a 20 weight oil but even then, it seems placebo.
If you are planning to change your own oil, 8 quarts of your preferred 0w20 synthetic will do. Mobil1, Amsoil, Shell Rotella GT, Castrol, Pennzoil/Quaker State, Redline, the list goes on. No need to over think.