I need to replace my rear cover for support on bearings. Car has 4.10's and is run, don't baby it. I know Ford says 75W 140, some guys are using 75W 90, recommendations? Car is stick, about 425 HP. Also, with using the friction modifier, how many quarts? Thanks.
I would stick with the 75W-140. Takes three quarts, and one bottle of friction modifier. If you use Royal Purple gear oil, you don't need the friction modifier; it's already in there.