I'm going to change the gear oil in my front and rear differentials in my 85 trooper soon but I don't really know that much about gear oil. Which brands do yall think are the best? More than likely I'll go synthetic unless its not worth the extra money. I need 75w 90 for the front and 80 w 90 for the rear and I was wondering if I could just use the 80w for both. Would it cause any problems?Also, I don't know if I need to add the limited slip stuff or not, do I?

I used Valvoline SynPower in both the front and rear diffs on mine. I just used the same weight in both 75w90 is fine to use in both of them. It also has LSD additives in it so you dont have to mix if you have LSD. If you dont it wont hurt anything.

If you don't have limited slip, you certainly don't need any additives.

I used Mobil1, but some have said (and I kind of agree) that synthetic is less valuable in non-combustion applications like differentials, transmissions and t-cases. Mobil1 in my transmission certainly made the shifting smoother.

If you go synthetic, you won't have a lot of choices with regard to viscosity. I don't think you'll find 80W90 synthetic. Even with the out of spec viscosity, the synthetics will work fine.

I'm kind of a synthetic nut--I even use synthetic grease in my wheel bearings and keep synthetic grease in my grease gun. My Trooper has been 100% synthetic for quite a while, and I'd like to think it's worth the (minor) added expense.

Same here pretty much, all Mobil 1 synthetics in my Passport.

I've not heard of any differential failure due to "bad" gear oil brand. Whatever is approved by SAE is OK.

But I personally use full-synthetic RedLine fluids everywhere around the car. I only use Mobil1 (as a cheap solution <img src="/forums/images/graemlins/smile.gif" alt="" /> ) as the engine oil, everything else is RedLine.

I use Mobil 1 in my rear diff only because of the ARB. Before that, I used the cheapie Coastal or other store brand (which is what I still use in the front). IMHO, its not worth dropping any extra coin on a stock diff. The 'average' vehicle owner probably never even changes diff fluid and how many people do you know that have had R&P failure?

what ever is cheapest

I'll second the what ever cheapest. It made it 20 years without it.

I'll third that. Has anyone here ever 'worn out' a propery set up ring and pinion?

Never worn gears out, but I have gone through a few seals here and there. If I was going to be concerned with something it would be preserving the seals the best way possible, not wear on the gears.