8-3/4 gear oil, conventional/organic or synthetic?

1971 - 1974 Mopars

  1. Hijinx

    Hijinx FBBO Gold Member FBBO Gold Member

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    Just had an auburn limited slip put in with all new bearings, seals and switched to 3.55 gears on a 8-3/4 489. The gear guy who did the work recommends synthetic, while Auburn recommends conventional with their additive.

    I am leaning towards Amsoil Severe Gear 80w90 synthetic, but wanted to hear everyone else's opinion. Anyone using synthetic? any noise or other issues?

    If you went with conventional gear oil, did you research any of the brands out there and which one did you pick?
     
  2. vance.dykes

    vance.dykes Well-Known Member

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    doc diff told me on my 8 3/4 power lock to use the additive he sells witch is motorcraft and conventional gear lube.
     
    Last edited: Mar 13, 2020
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    • Slap Stick

      Slap Stick Well-Known Member

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      Try Red Line Shok Proof. You can't hardly pick up a gear from a quick change with that stuff on there.
      I use it in the tranny and bevel drive of my bikes. Really improves shifting.
      And no limited slip additive needed.
       
    • 33 IMP

      33 IMP Well-Known Member

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      I would guess that Auburn knows their clutches and bearings better than your "gear guy".
      I had a friend (deceased from cancer, darn it!) who owned a one-man rearend shop. He worked on anything, but specialized in street/strip nine inch ford. He did my nine inchers, and my 8 3/4 with a locker.
      He would warrantee his work for six seconds/six inches, whichever came first, if the customer insisted on synthetic lube. He used Torco exclusively, usually the heaviest he could get, being here in cali. He also used the motorcraft supplement, depending on the limited slip used, and how tight he set the clutches.
      He felt synthetic was too slippery, didn't cling to the gears or cushion bearings. He never had a comeback using Torco.
       
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      • rrTor-Red

        rrTor-Red FBBO Gold Member FBBO Gold Member

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        Amsoil here
         
      • khryslerkid

        khryslerkid FBBO Gold Member FBBO Gold Member

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        If you have a new ring and pinion you don't want to use a synthetic. Just like breaking in a new engine, useing a synthetic won't allow things to break in correctly.

        Id use the 80/90 conventional with the Motorcraft friction modifier. You can buy it at your local Ford dealer.

        If Auburn won't warranty the limited slip with any other additive but theirs, get it from them.
         
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        • slepr1

          slepr1 Well-Known Member

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          Follow manufacturers recommendation.
           
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          • 451Mopar

            451Mopar Well-Known Member

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            With the new strange S60, I was told to use regular gear lube for the first 500+ miles before switching to synthetic.
            I usually use the Red Line Brand. It is supposed to already have the friction modifier in it. Never had any issues using it.
            For racing (and street) the shock proof gear oil may add a slight safety margin on launching the car?
            I called Red Line and the guy I spoke too said the shock proof is fine to use on the street, but may need to be changed more often than the regular synthetic gear oil?
             
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            • Cranky

              Cranky Banned Staff Member FBBO Gold Member

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              Did a pretty good amount of reading before sticking in Mobil 1 Synthetic LS into mine. Powr Lok w/3.55's in a Dana 70. Seems fine with the gears but have a slight chatter with the PL. I'm about to experiment with some STP additive. :D
               
            • dadsbee

              dadsbee FBBO Gold Member FBBO Gold Member

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              Wish I'd bought cases of it as a teen... Used my last bottles on the Bee, some 30+ years later and I'm out..
              beerestoration2017 1986.JPG
               
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              • Slap Stick

                Slap Stick Well-Known Member

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                If synthetic is "too slippery" to cling to the gears...why does it cling to the gears?
                 
              • Hijinx

                Hijinx FBBO Gold Member FBBO Gold Member

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                So no consensus on the subject. Been looking on vette forums and others and everyone seems to make valid points but it boils down to the same ole stuff... 2 camps one for one against, doesn't seem to be any quoted studies on the matter. Both sides have good success with conventional and synthetic. The two differences i see is that some using synthetic claim to hear chatter (not everyone just a few) the other difference is the synthetic lasts longer and reduces heat better.

                So probably going to use synthetic (with the auburn gear additive) and see how it goes, if it doesn't work out I'll try the conventional.
                 
              • khryslerkid

                khryslerkid FBBO Gold Member FBBO Gold Member

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                You never stated if you were using a new ring and pinion or not. If you are you don't want to use synthetic.

                Don't take my word for it, call or email the guy who builds them everyday.

                DoctorDiff
                (406) 833-4772
                doctordiff@gmail.com
                 
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                • khryslerkid

                  khryslerkid FBBO Gold Member FBBO Gold Member

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                  Your question is answered right on their website.

                  Q23. What kind of oil should be used with an Auburn limited-slip differential/ Auburn Select-A-Loc differential?
                  A23. Non-synthetic 80w90 GL-5 oil treated with Auburn Gear friction additive, part #504102 (also known as a friction modifier). See Catalog page 32. Three (3) ounces of additive will treat one quart of oil. GM or Ford limited-slip additive may be used. We do not recommend synthetic oil. Auburn limited-slip additive is packed in every box with the differential.

                  https://aftermarket.auburngear.com/maintenance_and_warranty/
                   
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                  • Hijinx

                    Hijinx FBBO Gold Member FBBO Gold Member

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                    Yes and no, according to most people Auburn Yukon etc rely on old compatibility tests and so stick with their original findings and recommendations, meaning they haven't really tested with the latest synthetics.
                     
                  • khryslerkid

                    khryslerkid FBBO Gold Member FBBO Gold Member

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                    There ya go, don't use the synthetic.
                     
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                    • dadsbee

                      dadsbee FBBO Gold Member FBBO Gold Member

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                      Maybe he wants to be a beta tester kid!
                       
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