355 Lslip, what are the best fluids to put in my rear differential? how much friction modifier?

 

Any synthetic 75w-140 fluid and (even if the fluid says it has FM in it, still use the Ford stuff).

You will get 10 different answers on exactly which gear oil is "best", but any synthetic will do in all truthfulness. Heck, even WalMart Supertech 75w140 syn gear oil is just fine.
Redline, Royal Purple, Mobil 1, Lucas, Valvoline, Pennzoil, Motul, OEM Ford, Torco, Supertech, any will work.

I used Lucas in mine. Why, it was the cheapest. Was going to use Supertech (~$11.35/qt at walmart), but I had a Amazon coupon and ended up getting the Lucas stuff for about $8/qt.

 

awesome, thanks