Front axle - D60 Monobeam from a 1991.5 F-350 Dually.

Rear axle - Ford 10.25" LS from a 1996 F-250

I know I need 8oz. of friction modifier for the rear. The owner's manual for my 1993 F-250 only lists Ford spec #'s for the fuild type. What weight should I be using for each axles? Do I need to run synthetic in the rear?

 

80w90 was a usual factory fill back then for rear ends. You don't need, but you can use 75w140 synthetic. Normally, gear oil don't get changed much, so a more expensive oil don't cost much more in the long run. The front was either 80w90 or 75w90.