All Mobil 1 grades are a mix of GRP IV (PAO) & V (AN and maybe esters), and many have a percentage of GRP III.
Mobil 1 still claims that their oils are "majority" GRP IV based, and I don't see why they would switch to a majority GRP III oil when they themselves are the largest PAO producer in the US.
From what I have read on the board, M1's 0w40, 5w40 tdt, 15w50, high mileage, and "0w" oils are considered to have higher PAO content than the mainstream grades.