I INSTALLED FRPP 4.10 GEARS LAST WEEKENS AND USED VALOLINE DURABLEND 80W-90. HOWEVER, AFTER LOOKING IN THE OWNERS MANUEL, IT SAID TO USE 75W-140 FULL SYNTHEIC. I DID USE 4OZ OF FRICTION MODIFIER. DOES ANYONE THINK I WILL HAVE ANY PROBLEMS.

 

IIRC, Ford used to require 80W-90 oil for their rears, and just recently switched to the heavier stuff. The rear design hasn't changed, so I'd say you're ok.

Also, many people remark on the robustness of the Ford 8.8 differential. I don't think you'll have a problem.

 

The heavier stuff is better in hotter climates or where there's more heat in the diff from performance usage.