Best Rear End Gear Oil

I have a 1993 GT Mustang. Street legal. Mostly used at the track.
New 3.73's, 31 Spline Moser Axles, Eaton Heavy Duty Posi.
What's the best gear oil to use?
Thanks

i hear that redline is great.. but than again most of everyone tells me theres nothing wrong with OEM stuff..

I want to give Royal Purole a try....but right now I have QS 80W-90 with the FM

I run....Mobil 1 Synthetic Gear Lubes

sorry if i'm jacking this thread, but is there much gain (ie decreased drivetrain loss) by changing gear oil? if so, how much? or is it just for better protection?

synthetic i've heard can gain for about 5hp, but that was coming from a guy that runs a high 8 sec front engine dragster...i doubt you'll feel a difference, but the only thing a quality oil will hurt is your wallet

~Mark~

Amsoil is a pretty good name also.

I use royal purple . I've tried others and would say that it seems to perform the best.

I wouldn't think you will feel anything from changing gear oil but it's just good to have clean fresh fluids...and when you have the cover off you can get rid of some metal shavings that may be in there

I run RP in mine.

I'd say pretty much any of the synthetics would be a good way to go with rear gear fluid. It seems to hold up a lot longer over time than the old-school dino juice. No gains or anything, just longevity.

FWIW, I run Mobil 1 in my diff.

i would say synthetics should be fine. however, when i installed my eaton posi, there was a tag in the box that said to not use synthetic oil. maybe the clutch plates don't like it? it said to use regular gear oil with the friction modifier. any eaton guys out there that know?

Yes, conventional oil not synthetic. Symthetic will cause clutch chatter.

holy crap! back from the dead!

i'm running (75-90 i think?) synthetic royal purple now, and no friction modifier, with and extra disc in my trac-lok. no problems so far, been running it about 2 months now....

Yeah, no kidding.

The factory stuff is hard to beat. Amsoil is some unreal stuff, I have seen it do things no other lubricant can in industry ASTM testing. Mobil is good, Royal Purple is decent, Redline is junk. For you guys running the Zexel Torsen and Eaton True-Trac, the F-series Superduty factory fill performs really well with these or any other type of mechanical differential not using friction materials (clutches, cones, ect.).

Personally I run a mix of honey, silly putty, bourbon and mustard. I have no gear whine and great traction. Only down fall is if your ridding behind me it smells like none other............... JK .........................I run Mobil 1 75w140 synthetic with friction modifer from Ford. Dont use any gear oil synthetic or non without the friction modifier. The FM is that nasty god aweful smelling blue **** in the 4 oz bottle. Dont get it on your hands, youll never get that stench out. Anyway AMSOIL and MOBIL make the best lubricants, use one of those and swing by the Ford dealer and grab a 4oz bottle of whale sperm.

Well from what i've learned so far, thats not always true. you want to run as little friction modifier as possible, as long as you don't get chatter. The royal purple has some sort of friction additive already in it. I had my gears installed by a mechanic that ONLY does performance ford work. nothing else, for the last 17 years & its mostly on mustangs. he recommended not to use the modifier, and i trust his work. i've heard the same before around here a couple times also.
no chatter so far, no more 1 wheel peels, and hooks dead straight. no complaints.

Redline synthetic with no FM here - works great.

I talked to Eaton tech today.
They do not reccomend synthetic gear oil with their posi units.
Coventional 80w-90 for the North Eastern part of the states.
Also, get the friction modifier from a Ford dealer. There is one that has a sea green color, that's the best one to get. And there is another that is an orange color, either one will work but the green one is the best I was told.
Fill 1/2 way with gear oil. then add 4oz friction modofier, then fill the rest with gear oil untill it flows out of the fill hole.
Clutch chatter when cornering after a drain is normal because the clutches are not fully lubed.
Go to a parking lot and do several figure 8's in each direction and that should get rid of any chatter.
This is what I was told at Eaton.
I thought I'd pass the information on to everyone.

I used Lucas in mine and you don't need a friction modifier either. I havent had any problems with it.