Im going to be doing axle seals and figured since it is going to be all apart i might as well put synthetic in. my question is, i have a bunch of mobile synthetic 75 90 sitting around that im not using. i know this isnt the "stock" weight according to my manual. i was wondering if it was ok to use and if/any difference it would make. i know i also have to use the slip additive in my rear end. thanks for any help.

 

I thought that weight sounds about right.

 

I just changed the differential lube in my Ranger and the owners manual calls out for 75W-90. I have a Torsen in my rear differential and the tag on it states to use only 75W-140 synthetic but that is only for the Torsen. I used 75W-90 in the front and if you have the factory Trak-Lok limited slip differential in the rear you should use the friction additive with the 75W-90 gear lube. The synthetic I used said is good for standard and limited slip differentials but I would still use the friction modifier additive with a clutch type limited slip differential.

 

80W90 is what i'd run just because thats what stock is. I don't see ANY 75w90 listed in the Ford workshop manual. Can't you just bring that to any store and get store credit then grab the appropriate weight fluid?

 

Yes, you are right. the specs do call out for 80W-90. I got the 75W stuck in my head from the 75W-140 called out on the tag on my differential housing. I am still using the 75W-90 for my front differential. I can't imaging how it could make much difference except to be slightly less viscous when cold. I think the 90 is the most important part of the call out. If Ford calls out 75W-140 for the rear differential 75W-90 should be good for the front differential.