Found an article about synthetic oils from 1978. Guess what weight the Mobil 1 was? The stuff that had a 25k oil change interval?
It was 5W20.
I wish I could get a can of that stuff for analysis now.
I think the original formula of Mobil 1 was also all PAO, with no esters, and caused a lot of engines to develop nasty leaks, which is why a lot of people to this day are still afraid to put in synthetic oil in a high mileage engine. They just think it's going to cause leaks.