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Gear head said not to use synthetic in diffs and this is why!!

Discussion in 'Maintenance & Upkeep' started by the phantom, Aug 7, 2012.

  1. dpeter

    dpeter Rockstar 3 Years 100 Posts

    Just read the article about/from Amsoil and I just got to say be careful what you believe or better yet, how you interpret what you read. A report prepaired for and pressumably paid for by Amsoil that puts its close competitors in an unfavorable light while their product seems to shine. Kinda like comparing golf balls and maker of golf ball "A" says of all the balls tested we were the only ones to have a blue stripe and all the others failed this test. The ONLY conclusion you can make is that golf ball "A" is superior. While the tests are likely accurate, I wonder if they are applicable and therefore relavent.
  2. Skippy

    Skippy Member 2 Years 100 Posts

    Yep, definitely worth taking a look for a different brand next go-around. The rear diff rarely gets pounded on, though, as I'm using stock sized tires and my off-roading is limited to 1 month a year (hunting season!) and then we're not exactly killing things going 5mph around forest service roads...
  3. Hidef

    Hidef New Member

    If your truck has the Eaton Locker (G80) the recommended gear lube is 75W90 Synthetic. The synthetic is recommend due to the use of clutch packs in the locker. Change the gear lube at the recommended intervals and you will never have an issue. The first time I dropped the rear end lube in my 2000 it came out black buy the third time it still looked new. I always used a semi-synthetic in the front dif and never had a problem.
  4. steved

    steved Former Member


    I ran RP gear oil quite a bit, I don't like their 75w90 MaxGear one bit, but their 85w140 MaxGear is some stout stuff. They must be two different blends.

    Even in my POS Subaru, the 75w90 didn't fair well, and these are in differentials that are maintained...this was the fourth fill on the Subaru, and it drained BLACK...the previous run of Pennzoil 75w90 Synthetic drained like new with more miles. Even in the same differential (Dodge 11.5AAM), the 75W90 would drain ugly after only 25k, the next fill of 85w140 drained looking new with 100k.
  5. Skippy

    Skippy Member 2 Years 100 Posts

    I wonder if the discoloration is actually due to contaminants that are harmful, or if it's something benign like the purple coloring they add discoloring due to heat.

    As with oil, black doesn't mean the fluid isn't performing exceptionally still.
  6. steved

    steved Former Member


    It drained like water (acted extremely sheared) and was causing the differentials to whine (keep in mind this is a 75w90)...not scientific, but observable. Changed fluid to a 75w140 synthetic (something I had on hand), the noise is gone, the car is noticeable smoother (there is a vibration missing), and my mileage is up almost 2 mpg. Considering this is a 140HP Subaru that is 100% highway driven, its not looking good for the RP.

    You could actually see "stuff" floating around in the drain oil...several differentials, several gear oils and fluid changes, same result with RP 75w90.

    I would have said it was a single differential thing, but with both differentials on the car and then on my truck? While the car has some miles on it, there was almost nothing on either magnet.
    Last edited: Aug 17, 2012
  7. Hidef

    Hidef New Member

    I have always used Mobil1 75W90 it works well and is reasonably priced. It never turned black like the factory fill but the second drop was dirty. I figured it was from the clutch pack. My 2000 used to lockup all the time were as the 2012 with traction control doesn't so I don't know what the rear-end oil will look like, I only have 1800 miles on it.
  8. steved

    steved Former Member



    I drained the OE fill on my 2012 and it was very metal flaked. And fairly black (probably clutch material). I refilled with M1 75w90. That was 4500 miles, I probably won't touch it until 25k
  9. 1st Synthetics

    1st Synthetics Epic Member 5+ Years 1000 Posts

    Some people just don't get it. There is no benefit conventional oil has over a synthetic in the diff's other than cost. I once had a discussion with a bike guy that works on Harley's doe=wn the road from me say he will never use synthetics in the twin cam's due to it is to slippery and it wears out the bearings?????? The bearings wear out due to the fact that Harley replaced Timkin bearings to flat roller bearings in 2002 or 2003 for cost reduction and they are junk. It has nothing to due with the oil.
  10. BoneHead

    BoneHead Member

    I didn't realize that changing the diff fluid was that important. I've got a 1977 Malibu with near 200,000 on the original diff and no fluid changes that I can remember. My Dad was the original owner so I know what kind of maintenance it has had during it's life. Maybe things have changed that much over the years, but my Grandfather, who was a Chevy mechanic from the 50's to 70's, never messed with the diffs unless it needed different gearing, not because something was worn.

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