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Differential Gear Oil

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  • Steve M.
    Very Frequent User
    • January 1, 2006
    • 256

    Differential Gear Oil

    Doing a frame off on a 1969 convertible and have finished cleaning and refinishing the differential casing. I am having a hard time finding a straight 90W oil. I can find plenty of 80-90W but not single weight 90W. Any thought on using the 80-90W with the GM Limited Slip additive? Thanks for comments/suggestion. Steve
  • Joe L.
    Beyond Control Poster
    • February 1, 1988
    • 43193

    #2
    Re: Differential Gear Oil

    Originally posted by Steve Morrow (45063)
    Doing a frame off on a 1969 convertible and have finished cleaning and refinishing the differential casing. I am having a hard time finding a straight 90W oil. I can find plenty of 80-90W but not single weight 90W. Any thought on using the 80-90W with the GM Limited Slip additive? Thanks for comments/suggestion. Steve

    Steve-----

    GM never specified 90 wt lubricant. What was specified was 80 wt OR 80-90 wt. Use GM #88863089 in conjunction with the posi additive.
    In Appreciation of John Hinckley

    Comment

    • Gary R.
      Extremely Frequent Poster
      • April 1, 1989
      • 1796

      #3
      Re: Differential Gear Oil

      Steve
      Any quality 80-90 wt will work, I use non synthetic Lucas 85-140 in my builds with the GM additive. The GM product Joe mentions will work fine. Many guys like to use a certain brand and type, some swear by synthetic but I haven't had any issue with the Lucas or GM oils.

      Comment

      • Kenn S.
        Very Frequent User
        • September 10, 2009
        • 173

        #4
        Re: Differential Gear Oil

        At the risk of sounding heretical, the local mechanic that recently rebuilt the diff for my '80 swears by the Ford additive. Says it sticks and lubes better. Given the sheer number of Corvettes they own and work on, I didn't have a reason offhand to question him.

        I had a bottle of the GM additive, but elected to test his theory. Once the car gets rolling, we'll see...
        -Kenn
        1970 LS-5
        1970 350/300
        1980 L-48
        2004 LS-1

        Comment

        • Joe L.
          Beyond Control Poster
          • February 1, 1988
          • 43193

          #5
          Re: Differential Gear Oil

          Originally posted by Kenn Scribner (14116)
          At the risk of sounding heretical, the local mechanic that recently rebuilt the diff for my '80 swears by the Ford additive. Says it sticks and lubes better. Given the sheer number of Corvettes they own and work on, I didn't have a reason offhand to question him.

          I had a bottle of the GM additive, but elected to test his theory. Once the car gets rolling, we'll see...

          Kenn------


          They may be exactly the same thing. GM does not now nor ever has manufactured this additive. I highly doubt that Ford does, either.
          In Appreciation of John Hinckley

          Comment

          • Steve M.
            Very Frequent User
            • January 1, 2006
            • 256

            #6
            Re: Differential Gear Oil

            Thanks for the information. I will go with a 80-90W and GM Additive.
            Steve

            Comment

            • Duke W.
              Beyond Control Poster
              • January 1, 1993
              • 15610

              #7
              Re: Differential Gear Oil

              The original spec oil was Mil-L-2105D, but that Mil-Spec was cancelled some years ago and replaced by the commercial spec SAE 80W-90 GL-5. commonly called "90-weight". Any brand is okay as long as the bottle has the API "donut" with that information.

              I don't think there is any difference between the limited slip differential additive sold by GM, Ford Chrysler, Mercedes, or any other manufacture that offers clutch type limited slip differentials.

              You can also buy it from NAPA for probably a lot less than any dealer parts department.

              Duke

              Comment

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