Kdaddy

Hey guys,

I recently discovered a leak on my rear differential, so I looked around and the fix seems pretty straightforward. Unbolt bottom bolts. Let the oil drain out. Remove cover. Scrape Gasket. Apply black rtv gasket maker. Tighten bolt back to 33 ft lbs. let sit hour. Refill through plug hole until it spills out. If I missed anything or if you have any addition tip, I would greatly appreciate it.

But here's where it throws me off. In my owner's manual it says to use 75w-140 synthetic and on the tag hanging from the rear differential cover it also says to use 75w-140. I wanted to use Royal Purple's 75w-140 http://www.americanmuscle.com/royalp...0-gearoil.html

...but it says recommended for 05+ mustangs and the fitment further down the page doesn't mention anything about 99-04. I even called Americanmuscle.com and the customer service rep says not to use it... This is all to confusing. I really only wanted to use Royal Purple, because it has the added friction modifier blended in. if you all could shed some light on the issue or recommend an alternative product I would appreciate it.

Thanks,

-Chris

Moonshine

Put in some 4.10's while you're at it! haha.

Umm does your car have a limited slip differential?

lizzyfan

Royal Purple 75w140 is good stuff, I use it so should you

cliffyk

Traction-Lock was standard on most (if not all) new-edge GTs.

The RP doesn't really have friction modifier "blended in", it's just that synthetic oil is already sufficiently "slippery" so as to not need it (the f/m makes dino juice more slippery to prevent the Traction-Lock clutch packs from binding up).

If you look at the specs in the owner's manual you'll see that the spec for fossil-based oil is 80W-90 with a reference to footnote ¹. The 75W-140 synthetic spec does not reference the footnote because the f/m is not needed.

hpwrestler220

Hmmm.. Interesting cliffy... I ended up using synthetic 75w140 oil in my rear and i also had some f/m put in... I got it anyway and then i asked if i needed to even get it since i bought synthetic oil and my mechanic said absolutely i needed the f/m... So idk. I am glad i bought my friction modifier.. it was only a couple extra bucks and my rear is silent

cliffyk

It also won't lock up as tight as it would without the f/m--but not so much that it really matters unless you are racing competitively.

If I were loading synthetic axle lube I'd put it in straight, and then should any popping or creaking crop up add f/m 1oz at a time as needed to meet the personal noise requirements.

A "trick" we used in the old days was to use less than the recommended amount of friction modifier, or even none on track-only cars, to make clutch type LSDs lock-up tighter. They'd bark, creak, and pop in tight turns--but lock up pretty tight at the strip on on that deserted strip of back road with the white-washed lines 1/4 mile or so apart...

Kdaddy

Thanks Cliffy! You've always been the one to answer my questions in the past and you're always correct

But, for some reasons my owners guide is written differently. It says to strictly use SAE 75w-140 synthetic rear axle lubricant for the 8.8inch axle. The SAE 80w-90 premium rear axle lubricant is mentioned, but only for use in the 7.5inch axle. So if i buy the Royal Purple linked above, I should be golden right? And I'll definitely hold off on the F/M. If I see a need, I'll go pick some up and put in 1 oz at a time. That's very clever

Thanks Again guys!

Kdaddy

Just got off the phone with a guy from Modular Depot. For once in my life i live close to somewhere cool! Getting 4.10s installed. Nothing like hitting 2 birds with 1 stone :P

hpwrestler220

I was chirping every gear at the track last night, 1-4, so my lsd is still going strong! *knock on wood*

ct_white_stang

Crap I think I put in 75X90?