what do you guys know for sure is group 3 or 4, that itself will be good enough
what is valvoline synpower and castrol syntec? those are the two oils i use in my vehicles
Valvoline Synpower is greater than 80% and less than 90% GP III. I do know that they have UP TO 12% GP IV, BUT, I do not know if that is 100 cSt GP IV add pack or 10 cSt GP IV base blend, or some of both.

I do know that from over a year of UOA research, in many, many areas, Valvoline is one of the most shear stable oils.