Hi , the old mechanic here.. My present '85 Vette 4+3 probably didn't specify Mobil One back then, though the previous owner used it since the owner before him and claimed it was fine..
I use Mobil One 5w30 in my wife's 2010 Terrain, with the Ecotech 2.4-Dir.Inj, VVT, H.O. 4cyl, but it's under warranty, and of course, it's my wifes' so.........
Anyway, my vette now has 167,000 miles on it. Autozone had Mobil One in 5w30, but had Valvoline Full H/D synthetic in 10w30 so I bought that and changed it that way.. (with a Fram Tough Guard Filter) and a new plug gasket.
In my day, with that many miles, I would have gone with much heavier Castrol GTX 20W50..or later with Castrol SynTec 20w50...
Also I've been told if the car has used synthetic oil for a long time and switched back to regular oil, seal leaks develop. Either way, I still feel uneasy planting the gas pedal with "piss thin" 10w30 oil on a high miles engine, even though synthetic.. What about adding STP oil treatment to it (showing my age again??)
Open to comments.. Thanks to you all..