Gear Oil for 8 3/4 with Suregrip

Engine, Transmission & Driveline

  1. Kristof

    Kristof New Member

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    8880C466-D09D-4CD9-86D1-FE3AA34C58CD.jpeg F19FEBC3-7248-45AE-B0D0-A6913D2F8371.jpeg 0EA387DD-1239-44EC-862C-C4451E0B19DF.jpeg Hi, i’m looking to replace the oil in the rear end of my ‘66 Fury.
    Question 1: I have 2 litres of EP90 gear oil, API GL-4, for classic car use. Can I use this or is GL-5 called for?
    Question 2: How much limited slip additive do I have to add (Mopar MS10111)?
    Any toughts?
     
  2. cbarge

    cbarge World Famous Barge in a Budget FCBO Gold Member

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    Never heard of that brand gear oil before.
    Is it for cars of the 20-30-40's?
    It appears to be a straight 90 weight not a multi viscosity type oil.

    Nothing special about the gear oil requirements for your car.
    80w90 and the conditioner is fine.
    GL5 synthetic 75w90 or 75w140 works too.
    Once you go synthetic you cannot go back to regular gear oil.
     
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  3. Kristof

    Kristof New Member

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    Thanks for your answer.
    Kroon Oil is a Dutch oil company, they have a big classic car range and their products are distributed in Belgium as well.
    It’s a mineral monograde oil that should be compatible with old seals to prevent leaks.
     
  4. stubs300

    stubs300 Senior Member

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    Doesn't it say how much to add on the bottle?
     
  5. Kristof

    Kristof New Member

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    The package says 2-5% by gear lubricant weight, which is quite a margin. I’ll probably go for 5%...
     
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  6. twostick

    twostick Senior Member

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    The last one I filled up I used 75w90 and the whole 4oz bottle of friction modifier.

    Kevin
     
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  7. MoPar Maniac

    MoPar Maniac Well-Known Member

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    Who says you can't go back to dino oil after using synthetic?
     
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  8. 3C's & a D?

    3C's & a D? Senior Member

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    I am wondering that too. And I had to look up the product mentioned because the can says "transmission oil", as suspected it's straight 90 weight and will be good. I also always use the full bottle of additive.
     
  9. MoPar Maniac

    MoPar Maniac Well-Known Member

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    It meets GL4 specs. If that rear calls for that oil Bob's yer uncle.
     
  10. cbarge

    cbarge World Famous Barge in a Budget FCBO Gold Member

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    I use a full bottle of conditioner.
    Although not necessary but have access to a hoist, i run the car at idle in gear on the hoist to circulate the conditioner.
    A good drive does the same.
     
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  11. CBODY67

    CBODY67 Old Man with a Hat

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    (that "can't go back" reminds me of something other than rear axle grease)

    I believe the FSM details which type of oil is needed (GL specs and viscosity), plus how much conditioner to use? I believe the GL spec refers to how much "white metal"/brass is in the particular gearbox (differential gears in this case)?

    I would prefer the multi-vis lube, even if it's just 85W90. With the original seals, or seals from that era's production, it might be better to stay with the "dino" lube. If the seals are "recent", then synthetic might not degrade them (softening).

    Some modern lube state they work with limited slip axles, as if they build ONE lube for everything, now. But I also suspect this also relates to the change to the governor-controlled clutch lock-up (unlocked until it locks) rather than the earlier style with the springs applying the clutches (locked until it unlocks -- which needs the additive to smooth clutch slippage during turns). But later on, GM advocated using their additive with the "unlocked until it locks" PTrac units. However large their additive bottle was, one bottle per rear diff lube change.

    Many OEMs have used syn rear axle lube for about 20 years or so. But it still needs changing every so often (per FSM information). But for the OLDER axles, it might be best to stay witn modern versions of the original-spec lubes and ltd slip additives.

    Just some thoughts,
    CBODY67
     
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  12. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    Straight 90 is the old mineral weight for transmissions. New synthetic is considered to be a 50 weight. They are pretty much the same at 60-70°f, the lower the temp the synthetic flows better or more like 50w hence the rating, in the name of fuel mileage. It would probably be fine to use. Definitely use the additive with that stuff, most 85-90 gear oil you buy for a rearend is good for limited slip cone type, clutch type must use additive, and not be synthetic.
     
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  13. akiran

    akiran Member

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    The only suitable grade for hypoid gears is API GL-5. I'd prefer viscosity SAE 80W-90. No matter, is it mineral oil or semi synthetic.

    I used to run a car service and repair shop, for modern but also for older vehicles. I was a retail seller for UK brand Millers Oils, founded 1887, so I think they have some experience for they Classic range.

    Few basics: API GL-4 is for bevel gears with lighter loads. Also suitable for yellow metal (bushings etc). Usually used in gearboxes, RWD transmissions and older rear ends.
    API GL-5 is for hypoid gears with heavier loads. Because hypoid gear tooths are making a sliding contact, the pressure is much higher. That's why GL-5 have additives that give the distinct smell for it. These additives make it unsuitable for yellow metals.

    These older gears have a little bit wider tolerances than modern. A thicker oil viscosity fills better a little bit wider gap between gear teeth, which prevents noise and abnormal wearing. Modern synthetic oils and lighter viscosities are for tighter gear teeth gap.
     
    Last edited: Mar 20, 2019
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  14. Kristof

    Kristof New Member

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    Thanks for your replies!

    I’ve just bought some Castrol Syntrax Universal Plus 75w90. (As per your comments decided to use a more modern multigrade oil). The oil can be mixed with mineral oil and should not degrade any seals.
    I’ll also add a full bottle of conditioner.
    Enclosed you can find an excerpt of the FSM with the original requirements for completeness.

    Thanks again!

    50226ACA-34DC-4163-8CF4-FD90551B5361.png
     
  15. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    I can't really agree with all that. Backlash is backlash it is set into the gear teeth to compensate for thermal expansion, it has no idea if the oil is synthetic or mineral (which is a misnomer in itself they are all made form the stuff they pump out of the ground just different selection of base stock and additives). Tolerances are tighter because mass machining and material recipes are improved from 50-60 years ago. The lighter wieghts listed for synthetic oils are because they flow better at colder temps, thus thus saving fuel, but because of selection and additives the protect as well as thicker mineral oil, in the heat.
    As I mentioned above do not use synthetic in a clutch type sure grips. 1966 original will have clutch type.
     
  16. MoPar Maniac

    MoPar Maniac Well-Known Member

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    Synthetic will not soften seals. If it did the seals would work better.
    What happens is the molecules of the oil are linear and get past old seals easier. If it happens go back to dino.
    In my bikes I use Red Line shock proof heavy 90-140, aka strawberry milk shake. Used it in the race car quick change rear end. Can't hardly grab the gears to remove them.
    Gets past the paper gaskets in the tranny just enough to be annoying. Use dino and all is well.