2008 Mustang GT Rear Gear Lube

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.

It works great with the 4.10s, I had the 4.10s first then went NOS. I was going to go supercharger, but I figured if I was going to spend the 5-6K there, with NOS I get 4K to use for other stuff. Right now I run the 100 shot as I am working on a new Race Tune and have to change my fuel pump to increase the flow rate before I move up with the jets. I also lightened the car, so the combination is pretty good. Killer 60 foot time, hoosiers big help there. I suspect I will lose the clutch very soon.