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Royal Purple gear oil + friction modifier?

Discussion in 'The Greasy Spoon' started by xtreme_exploder, Nov 30, 2015.

  1. xtreme_exploder

    xtreme_exploder Member Established Member

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    I ran Royal Purple 75-140 full synthetic gear oil in my rebuilt 03 Cobra differential since day one. The clutch packs did not last more than 5,000 miles before they burned up enough to easily turn the wheels in opposite directions by hand when the car is jacked up. I did not add friction modifier as the bottle explicitly stated that it was designed for limited slip diffs and no additional additives were needed. Everyone says adding friction modifier isn't necessary, but is this really true? Has anyone added the Ford friction modifier to the Royal Purple oil?
     
  2. SID297

    SID297 OWNER/ADMIN Staff Member Administrator

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    I have used RP 75w140 for the last 20,000+ miles in an F-350 with no FM and no ill effect.
     
  3. Joewee500

    Joewee500 Member Established Member

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    I will be using the RP 75w140 with a gear install next week. I bought some friction modifier as well and will add it just in case.
     
  4. 04sleeper

    04sleeper Runs On "Liquid Gold" Staff Member Super Moderator

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    I am running it in my TT 13 GT and is working fine. I would look into the install or something else as the culprit.
     
  5. RedRocketMike

    RedRocketMike A Member Well Known Established Member

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    You don't add it to Royal Purple, or any GL5 gear oils that I know of. They already contain about 3%. Drain and refill it, that fluid is ruined. Not sure if your pack are ruined, maybe just not able to operate with so much FM.
     
    Last edited: Jan 6, 2016
  6. adrian3019

    adrian3019 Member Established Member

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    He didn't use FM,
     
  7. RedRocketMike

    RedRocketMike A Member Well Known Established Member

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    I guess I misread, don't recall after this long. Someone in here did add it though.

     
    Last edited: Mar 22, 2016
  8. xtreme_exploder

    xtreme_exploder Member Established Member

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    I'll be installing a trutrac to avoid this problem. I'm thinking the stock friction style diff doesn't last under high power road course abuse.
     

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