diff oil: 80w90 or 75w140?

YELOSNK

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I've seen folks state both 80w90 and 75w140. Which is the best choice? If it matters, I have 4.56s and this car is solely for backroads bashing with (hopefully) a few track days per year sprinkled in going forward.

And past that, any reason to pick one or the other between RP, Amsoil, and Red Line?
 

01yellercobra

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My understanding is it comes down the clutch packs. The 03/04 calls for the 75w140 because it's synthetic which is supposedly better for the carbon fiber clutch packs. Since the 99/01 don't have carbon fiber (unless you upgraded) either one is fine depending on your climate.

All that being said I've run both in my car with no adverse reactions. But when I ran the 80w90 the diff had about 100k on it. Since replacing the clutch packs I've had 75w140 in there with the recommended friction modifier.

I originally put in the O'Reilly's brand with the plan of swapping it out for Lucas after a few hundred miles to flush out whatever particles might be in there from the rebuild. But I ran into an issue where my thread sealer worked a little too good and I almost broke a bit trying to get the drain plug out. So the generic stuff is still in there and I don't have any weird noises yet. That was close to 15k miles ago.
 
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YELOSNK

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Gotcha, thanks. I have not yet rebuilt the diff, though I'm sure that's in my future. I'm guesing the 75w140 would have some marginal parasitic affect, but beyond that is it going to stand up better to the heat on those few track days?
 

01yellercobra

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I think for track days it's going to be marginal about which lasts longer. Either one is going to get cooked because of exhaust right under it. Depending on how serious you plan on being you'll probably want to consider an external cooler. If you're not going to be serious then just check it regularly and change as needed.

Mind you, I'm not a track guy so these are just my thoughts. YMMV.
 

YELOSNK

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I think for track days it's going to be marginal about which lasts longer. Either one is going to get cooked because of exhaust right under it. Depending on how serious you plan on being you'll probably want to consider an external cooler. If you're not going to be serious then just check it regularly and change as needed.

Mind you, I'm not a track guy so these are just my thoughts. YMMV.

Thanks! I don't anticipate being serious about it anytime soon. Just want to do 2-3 a year and have fun.
 

offroadkarter

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80W90 is what Ford called for when these cars were in production, all 8.8's got superseded sometime later and switched to 75W140. Either option will work, but if you went to a Ford dealer today and had them drain/fill the diff, they'd put 75W140 in.
 

YELOSNK

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Thanks. I'm thinking I'll just split the difference and go with the severe gear 75-110. I've read on here that with amsoil one needs to add friction modifier, but on Amsoil's page for their own modifier it says "AMSOIL synthetic gear lubes do not require this additive. ". Which is it?

P.S. Beautiful E39 M5, offroadkarter! My friend had one like it that sadly got totaled. He now has an E63 M6, I have an F10 M5.
 

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