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Hey folks. Gotta question. On my 07 fj, ever since I purchased a year ago you can hear slight noise coming from rear differential. My mechanics store says it's the Pinion bearing. I was wanting to drain gear oil and replace w Royal Purple synthetic oil. The mechanic says doing that could actually cause it to fail sooner. I don't see how it could do anything worse to it, if not actually be good for it. I'm not going to touch it untill I hear from you all. Thanks a bunch.
 

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First, Changing the gear lube in the rear diff won't 'hurt' it as long as you use the correct product. Second, I've had a gear whine that is sometimes very noticeable on my '07 since day one. The service mgr. said it was normal, I disagreed but's it's now got 157K and it's actually gotten a better over time. Either that or my hearing has gotten worse. I've dumped the diff lube twice and the only difference I noticed is that it rolls better downhill when it's really cold outside, as in less resistance. No difference in the noise. But I'm convinced now it's not a problem.

Change away with no fear but don't expect too much.

DEWFPO
 
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Everyone is entitled to an opinion and it doesn't mean they are right but I switched from Royal Purple to Lucas Oil and it reduced my clunking noise by a substantial amount. I switched based off of this recommendation by ECGS.

 

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I could certainly see how a thicker gear lube like 85W140 could reduce a clunk when hot.

DEWFPO
 

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Change the oil, specially if you do not know the service history. It is a 10 minute job and it will not hurt or make things any worse. I just did mine with Mobil 1, 75w90, but after reading the ECGS comments definitely worth going with the Lucas 85W140. Get the crush washers and make sure you remove the fill plug first. Mine had a pretty good whine with load at noticeable speeds above 55mph before the oil change. After the change, the sound is still there but definitely not quite as loud. The sound is very noticeable between 55 and 70 mph.
 

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I always tell people to go with what their diff builder recommends. And if it's still an OEM rear, whichever satisfies the OEM recommendation. I subscribe to the ECGS recommendation, of cheaper oil and more frequent changes. The top shelf stuff is race car stuff and usually teams change them more frequently due to the hard runs (seeing higher temps for extended periods). This doesn't make it any better or worse IMO, but the belief that it's any better for your use is not the case. If it's already on it's way out, a diff fluid change won't make a difference. If it's a worry about a fluid change prematurely wearing, I'd stick to something less fancy with less additives as you don't know how they'll react. Replace with Hypoid API GL-5 and as DEWFPO suggested, don't expect much.
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Everyone is entitled to an opinion and it doesn't mean they are right but I switched from Royal Purple to Lucas Oil and it reduced my clunking noise by a substantial amount. I switched based off of this recommendation by ECGS.

There are a number of statements in there that don't seem to match up with what I have read about how oil in general and synthetic oil in particular acts. For example, one of the most desirable characteristics of true synthetic is that it doesn't break down with heat as readily as dino oil, opposite of their anecdotal observations. Just because it seems thin doesn't mean it has broken down; synthetic flows much more readily than dino.

But as with most of these discussions, the fact that you change it and make sure the level is correct is far more important than which oil you use as long as it meets spec.

FWIW I changed mine at 100k, and now that I am wheeling it I will change every 30k.
 
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