Royal Purple gear oil + friction modifier?

xtreme_exploder

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I ran Royal Purple 75-140 full synthetic gear oil in my rebuilt 03 Cobra differential since day one. The clutch packs did not last more than 5,000 miles before they burned up enough to easily turn the wheels in opposite directions by hand when the car is jacked up. I did not add friction modifier as the bottle explicitly stated that it was designed for limited slip diffs and no additional additives were needed. Everyone says adding friction modifier isn't necessary, but is this really true? Has anyone added the Ford friction modifier to the Royal Purple oil?
 

xtreme_exploder

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Yes, I'm aware. But like I was saying has anyone added the Ford stuff to the oil? Maybe the Royal Purple additives aren't enough or aren't the right ones.
 

Mach828

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I run conventional without limited slip modifier. I also custom pack my LSDs.

My stock unit lasted 2 trips to the track, my current one has lasted 100s of passes and years of boosted abuse on sticky tires.
 

jrichber

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I had some problems with the clutch packs in my truck so I talked to a friend that has a tranny/drivetrain shop and told him after I changed out the fluid hoping to correct the problem, it only got worse. He told me whenever he does clutch packs he uses synthetic lube with the supposed additive in it and also adds a bottle of friction additive per quart of gear oil. Guess its more of a better safe than sorry type thing. After having the problems I did with my truck I will be doing the same.
 

xtreme_exploder

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I run conventional without limited slip modifier. I also custom pack my LSDs.

My stock unit lasted 2 trips to the track, my current one has lasted 100s of passes and years of boosted abuse on sticky tires.

Are you running an IRS with carbon clutches? I run conventional without modifier in my Explorer, seems to be just fine. But that's a solid axle (runs cooler) with the standard friction material.
 

xtreme_exploder

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For now I think I'll repack/shim the clutches if they're not too worn and run Royal Purple with the Ford additive. Just to experiment, as it'll be cheaper than a new Truetrac (unless someone's got one cheap lol)
 

Rich'sTRsvt

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Did you feel binding in the rear end when doing tight turns? If not then the friction modifier was working fine, it is added to allow the clutches to slip during normal driving. If there wasn't enough friction modifier you would get binding or popping when doing tight turns.
 

xtreme_exploder

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I never felt anything, I thought the same thing. Just don't know why the clutches would have loosened up so much in such a short amount of time.
 

JoeCobra

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We've run into this problem before with Ford truck differentials in the shop i work at. We use Valvoline Synthetic gear lube which states the same thing as the royal purple, that the additives are inside already. However with the Fords for some reason we have to add the Ford Friction Modifier part# XL-3 to avoid the issue with the clutch packs. I recommend using that friction modifier with your synthetic fluid.
 

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