far34

I was wondering what type (weight) of gear oil, and the amount needed to Change the rear diff fluid in my 1991 f150 4x4. I am almost positive that she has an 8.8 Also, the guy at the parts store said something about an additive I should use too, and it even showed reccomended on the screen as a side note. Also, anything I should check up on while The cover is off, besides visual inspection for normal gear wear?? Thanks yall

Austin97

75w-140 synthetic and you only need friction modifier if you have limited-slip.

joshtowal

I'd recommend an additive too if I got commission from it. Go by what the manual says and not the guy at the local parts store. If you really want to know, call out to the parts department at a few different dealerships just to be sure.

1990-5.0-4x4

x2 going by the manual. unless you plan on abusing your rear diff, then i'd go with something a little thicker, but thats about it. i've heard about some guys using lucas oil stabilizer in their diffs and transfer cases, but im assuming thats just a personal choice kind of thing.

farmer12

If it's an open diff just run 80-90. An if ya have the limited slip buy the friction modifier to add to it. Just look at the tag that's bolted to the cover. If it's a limited slip it will have LS on it somewhere.

klricks

There also should be an axle code on the door jamb sticker. Find an on-line decoder and that should tell you if you have LS or not.

sylver91

Rear need 5.5pt of fluid, 80-90 or 75-140 and 4oz of friction moddifier if needed, and since you're doing the rear may as well do the front too which needs 3.6pt for the Dana 44

chris302

I used the lucus for mine works great and it is only like 5 bucks more and that was 30k ago still doing great but when u change it make shure to do a good inspection of your intenals

1990-5.0-4x4

did you use only the lucas oil stabilizer, or did you do a mixture of the 2? this summer i'm planning on doing front and rear diffs and transfer case with it, probably a 50/50 mix, at least in the transfer case anyways. i'm hoping to do some decent off roading this summer

far34

I bought some Royal Purple for it. Haven't changed it because the weather has sucked down here and I will have to do it in my driveway. But my gosh that RP is EXPENSIVE. my truck is so old, I am wondering if I just jumped the gun on it and should just put plain gear oil in it. Its not like my truck is high performance or anything. I am a conservative driver, b/c I am a BROKE college student at the moment, so The diff doesnt see any abuse.