I'm going to be changing the oil in both the front and rear diffs (7.5 non LS, don't remember the gear ratio). I bought this truck a few months ago and I don't know if it's ever been changed.

I bought some 75-90 synthetic for the rear. It recommended it in the owners manual. However I've read where Ford now recommends 75-140 in the rear diff.

Will using the 75-90 hurt anything? Will it matter if it's a full synthetic?

 

You posted this 3 weeks ago, but, No, you don't need a 140 weight in the rear. A 75W-90 is fine. You don't even need to add a Friction Modifier since you're is an open.

 

I always have and always will run 80W90 as thats what it says in the ford workshop manual.

 

Yeah that's what I ended up using. The old fluid looked like chocolate milk. Now it has nice honey colored fluid in it.

 

Good! I need to do mine before the winter, I do it twice a year for the hell of it since its cheap