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Rear Differential: Friction Modifier with Synthetic Fluid?

sfbayjay

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City, State
Cleveland-ish, OH
Year, Model & Trim Level
2002 XLT SOHC 4L V6
To Add, or Not to Add. That is the question.

I've had all kinds of issues with my '02 XLT's rear differential (3.73 Limited Slip) over the years, including a clutch pack replacement and partial rebuild. I'm kind of babying it now, changing fluid every 30K miles or so.

I bought some Royal Purple 75W-140 Max-Gear full synthetic for my upcoming changeout and I read that it has friction modifiers built right in. Handy if true, right?

I know Ford recommends adding 4 oz of mysterious XL-3 modifier to their Motorcraft synthetic 75-140 for the rear diff.

I emailed Royal Purple and their tech folks are of the opinion that no XL-3 is needed if you use RP Max-Gear. From their email response to me:

"...yes it does already have an appropriate dose of friction modifier included in the Max Gear formulation. No addition friction modifier should be needed."

So... ...??? What to do here? I'm tempted to just add the XL-3 anyway to be safe. It's only a few bucks for a bottle from my local Ford dealer. Don't want to risk damage to clutch packs, as they were expensive to have replaced!

Any opinions out there? Will adding an additional 4 oz of FoMoCo XL-3 compromise the effectiveness of the RP Max-Gear?
 
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dwcopple

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July 22, 2011
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bay city, MI
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'04 Mounty
not needed. I did royal purple alone and all the chattering disappeared.
 
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Exproblems

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Orchard Park, N.Y.
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'05 Explorer XLT 4.0L
>>>>>>>>>>>>>>>
To Add, or Not to Add. That is the question.

So... ...??? What to do here? I'm tempted to just add the XL-3 anyway to be safe. It's only a few bucks for a bottle from my local Ford dealer. Don't want to risk damage to clutch packs, as they were expensive to have replaced!

Any opinions out there? Will adding an additional 4 oz of FoMoCo XL-3 compromise the effectiveness of the RP Max-Gear?
>>>>>>>>>>>>>

You answered your own question here, add it , because it only costs a few bucks for the tube of friction modifier, plus you already know that you have a limited slip diff and Ford requires it in that type of diff.
 
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