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Thread: Which diff oil to refill the pumpkin?

  1. #1
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    Which diff oil to refill the pumpkin?

    Heya all,

    Planing diff flushes for the 94&95, and I had a few questions for the group.

    1- Reading the owners manual it calls for what I think is 90 weight oil (PN: XY-90-QL). Is this the same as 75w-90? Of course, I also did some reading and folks seem to prefer 75w-140. Is this better for higher mialage cars since its thicker (same concept as thicker motor oil)? Both my rides are 150k+ miles. I'm interested more in longevity and good operation, rather than super duper performance since my cars see no track or circuit action.
    2- I found 75w-90 & 75w-140 in both, motorcraft & royal purple. Any preference between the two? And why?...just so I can have the info for my future reference , or, my book of knowledge, as a good old friend used to say.
    3- Will royal purple, if I decide to go with that, still need friction modifier? I know using the motorcraft the manual says I have to but not sure about royal purple.


    Thanks for any help!
    Current: 1993 SC 5.0L Auto Red, 1994 SC 5-speed Teal, 1995 SC 5-speed Chameleon Blue
    Old: 89 SC auto Twilight Blue, 89 SC stick Titanium, 90 35th stick, 92 Sport 5.0L stick, 94 SC stick Black, 1994 SC Auto Teal
    Also: 1992 Feature Mustang Convertible 5-speed Vibrant Red, 1991 BMW 850i 6-speed Red, 1994 BMW 850CSi 6-speed Red

    www.mamn12.com for your mid-atlantic mn12 needs!

  2. #2
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    80W90 is what came in our cars from the factory. Some prefer to use the 75w140 because its fully synthetic and interchangeable with conventional 80w90. I refilled with synthetic 75w140 Motorcraft last time. And although the back of the bottle claims friction modifier may no longer be needed with some traction-lok differentials when switching to synthetic, mine still needed a bottle.

  3. #3
    Join Date
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    I recommend the 75W140. Just like with a motor oil, the lower number is the viscosity when cold, and the higher number is when hot, so under normal driving conditions, the 75W140 will be close in viscosity to the 75W90, but if you do wind up heating up the diff and needing the added protection, the thicker 75W140 will protect the diff better. There is nothing wrong with Royal Purple, but I usually use the Valvoline 75W140 synthetic because it is still a name brand, and only like $13/qt, as opposed to $18 for the Mobil 1 or $22 for the Royal Purple.

    As for the friction modifier, don't bother. At 150K miles, the clutches are well worn in, and have been soaking in gear oil with the friction modifier for quite some time, so you won't have any chatter without it.

  4. #4
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    Assuming it hasn't been changed since it left the factory - drop by your local Ford dealer and buy a bottle of their track-lok diff treatment and whatever weight fluid they recommend...nothing fancy.

  5. #5
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    I anyone using Red Line 75W90?

  6. #6
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    Thanks all for the input. I'm leaning towards royal purple for the rear end. I tend to gravitate to motorcraft for the fluids that are easy to replace...but for the M5R2 fluid and rear end which are a lot less frequently changed, I will probably go with premium stuff.
    Current: 1993 SC 5.0L Auto Red, 1994 SC 5-speed Teal, 1995 SC 5-speed Chameleon Blue
    Old: 89 SC auto Twilight Blue, 89 SC stick Titanium, 90 35th stick, 92 Sport 5.0L stick, 94 SC stick Black, 1994 SC Auto Teal
    Also: 1992 Feature Mustang Convertible 5-speed Vibrant Red, 1991 BMW 850i 6-speed Red, 1994 BMW 850CSi 6-speed Red

    www.mamn12.com for your mid-atlantic mn12 needs!

  7. #7
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    Wintersville, OH
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    Mobil 1 75w90 here, already has the additives in it for Trac lock.
    Making woosh sounds since 2010!

  8. #8
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    I put Royal purple in my 95 5 speed and had difficulty getting it into reverse and then into 1st at times. When I changed it back to the Motorcraft I didn't have any problems.

  9. #9
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    Quote Originally Posted by fstcoup View Post
    I put Royal purple in my 95 5 speed and had difficulty getting it into reverse and then into 1st at times. When I changed it back to the Motorcraft I didn't have any problems.
    This happened when you used royal purple in the diff?
    Current: 1993 SC 5.0L Auto Red, 1994 SC 5-speed Teal, 1995 SC 5-speed Chameleon Blue
    Old: 89 SC auto Twilight Blue, 89 SC stick Titanium, 90 35th stick, 92 Sport 5.0L stick, 94 SC stick Black, 1994 SC Auto Teal
    Also: 1992 Feature Mustang Convertible 5-speed Vibrant Red, 1991 BMW 850i 6-speed Red, 1994 BMW 850CSi 6-speed Red

    www.mamn12.com for your mid-atlantic mn12 needs!

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