what to replace differential fluids with?
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This is a discussion on what to replace differential fluids with? within the Transmission & AWD forums, part of the Tech & Modifying & General Repairs category; im at a little over 40k and stil on stock differential and transmission fluid all around. what is a good ...

  1. #1
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    what to replace differential fluids with?

    im at a little over 40k and stil on stock differential and transmission fluid all around. what is a good replacement? preferably something synthetic
    It is better to have others think that you are a fool than to open your mouth and remove all doubt


    02 WRX 44k - sold 2/28/5
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    Registered User scooby24's Avatar
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    I just asked this question to Redline

    75w90ns for the tranny

    75w90 synthetic for the rear diff.
    05 Crystal Gray STi
    304 hp 353 lbs on 92 octane ------> Mustang Dyno
    12.6 @ 109.2 > new numbers coming soon.

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    Moderator fengshui's Avatar
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    Many folks also run Royal Purple in their tranny. At 30K miles, I put Royal Purple in the tranny and Redline in the rear diff and it take much less time for the tranny to not bind up on very cold mornings. Shifting has never been smoother.
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    i put amsoil syn 75w90 in my tranny and diff a couple weeks ago. feels great.
    Vanitas vanitatum...et omnia vanitas

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    so basically any 75w90 will work for the tranny and rear diff? what about the front diff...or is that covered right in with the tranny?
    It is better to have others think that you are a fool than to open your mouth and remove all doubt


    02 WRX 44k - sold 2/28/5
    96 Corolla 128k
    92 Corolla 160k
    88 Accord 208k
    90 Voyager 242k

  7. #6
    Registered User scooby24's Avatar
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    Just remember any normal 75w90 synthetic fuild is going to be a GL-4 fluid. Our trannies, being AWD and having a differential in the tranny need GL-5 to truly be safe. GL-4 will break down under extreme pressure whereas GL-5 will not. Also a normal synthetic will be very slick, which is good for the gears, but is bad for the synchros. Synchros need some friction to properly allow for smooth engagement of the gears.

    75w90ns is a GL-5 rated synthetic that has friction modifiers which help the synchros out. It is basically a fluid redline designed to be used in cars like ours which undergo extreme conditions and is still going to be safe on the synchros.

    Most straight synthetic tranny fluids will appear like a godsend for a little bit until it takes it's toll on the synchros and they will start to fail. Had it happen on 3 cars before I finally learned my lesson.
    05 Crystal Gray STi
    304 hp 353 lbs on 92 octane ------> Mustang Dyno
    12.6 @ 109.2 > new numbers coming soon.

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    so is the redline fluid good for our stuff? I think you kinda partially answered my other question in your response and that wasif the front differential is part fot he tranny and does not require a seperate fluid drain/fill?
    It is better to have others think that you are a fool than to open your mouth and remove all doubt


    02 WRX 44k - sold 2/28/5
    96 Corolla 128k
    92 Corolla 160k
    88 Accord 208k
    90 Voyager 242k

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    Registered User scooby24's Avatar
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    Quote Originally Posted by JeremyL
    so is the redline fluid good for our stuff? I think you kinda partially answered my other question in your response and that wasif the front differential is part fot he tranny and does not require a seperate fluid drain/fill?

    Front Diff, Center Diff, and tranny all use the same fluid. This is why the GL-5 fluid is so important. When the fuild breaks down the gears weaken as do the diffs.

    Yes redline 75w90ns is the best I've heard. I've also heard some people adding 1 quart of synchromesh along with the stock fluid and that helps for driveability but I'm not sure about longevity going that route.

    Personally I trust the people at redline and if they say 75w90ns then that's what I'll use
    05 Crystal Gray STi
    304 hp 353 lbs on 92 octane ------> Mustang Dyno
    12.6 @ 109.2 > new numbers coming soon.

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    Moderating U! KurleeDaddeeWRX's Avatar
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    When I had my WRX, at around 30k I put in Redline Shockproof Super Lightweight Trans/Diff fluid and I was impressed. Shifts became buttery smooth and a lot of the tranny/diff noise went down.

    In my SRT-4, since ATF is used in their manual transmissions, I am using AmsOil ATF+4. It helped a little bit by making shifting easier, but it wasn't as much as an improvement as I felt with the WRX and the Redline.

  11. #10
    Registered User scooby24's Avatar
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    Quote Originally Posted by KurleeDaddeeWRX
    When I had my WRX, at around 30k I put in Redline Shockproof Super Lightweight Trans/Diff fluid and I was impressed. Shifts became buttery smooth and a lot of the tranny/diff noise went down.

    In my SRT-4, since ATF is used in their manual transmissions, I am using AmsOil ATF+4. It helped a little bit by making shifting easier, but it wasn't as much as an improvement as I felt with the WRX and the Redline.
    Redline recommended against using their shockproof. This is what they told me

    Greg,

    The ShockProof Heavy does perform extremely will in the Harley transmissions but can be too slippery for the synchros in a typical manual transmission/transaxle application. We use it in speciality applications such as the Harley and BMW motorcycles as well as various racing applications. It can be too slippery for synchro applications, the cold flow characteristics are better with some of our other products and due to the unique appearance wouldn't be advised while under warranty.

    In your WRX transaxle I would recommend the 75W90NS, it will perform well and offer good shiftability. The 75W90NS is not harmful or too slippery for the synchros nor is it corrosive, I wouldn't expect any issues with synchros from using the gear lube.

    The center differential/transfer case shares the same gear lube with the transaxle and the front differential as well.

    In the rear differential I would recommend the 75W90.

    Regards, Dave
    Red Line Oil
    05 Crystal Gray STi
    304 hp 353 lbs on 92 octane ------> Mustang Dyno
    12.6 @ 109.2 > new numbers coming soon.

  12. #11
    Moderating U! KurleeDaddeeWRX's Avatar
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    Quote Originally Posted by scooby24
    Redline recommended against using their shockproof. This is what they told me
    Thanks for posting what Redline had to say about it. But if I ever go back to a Subaru, I will put more Redline Shockproof Lightweight in my trans and diffs. I had none of the problems stated in that email. No grinding gears or anything. The things he describes is what people on here experience with Mobil 1 synthetic trans/diff fluid. Redline Shockproof is good stuff.

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    Moderating on the run! Big Sky WRX's Avatar
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    I also used the Redline shockproof (LW) w/ very, very good success- as have numerous others, I've only got 7,000 miles on my new wagon so still have oem fluid in it.

    I think I've read accounts w/ folks that have used both NS and shockproof and the shockproof proved better. I haven't tried the NS so I'm reluctant to poo poo it, maybe I'll give it a try when I'm ready to swap out fluids.

    The shockproof does come in a rather unique color- the LW is like "Hulk" green, so it may not be the best as far as waranty issues are concerned.

    I can tell you to avoid Mobil 1 ist is too slick, downshifting (even w/ perfect rev matching) became a chore- shockproof- butter

    I may give the NS a shot, if it doesn't work- I know what will

    Big Sky

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    it may have worked well on a short term testing basis but has anyone run this stuff for more than 50k?
    It is better to have others think that you are a fool than to open your mouth and remove all doubt


    02 WRX 44k - sold 2/28/5
    96 Corolla 128k
    92 Corolla 160k
    88 Accord 208k
    90 Voyager 242k

  15. #14
    Registered User zoinks's Avatar
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    I put in the NS back in the spring, and ever since then 1st and reverse grind worse than before. It's not all the time but definitely more than it did. I may try adding a bit of syncromesh to the mix to see if it helps. Also, I could be wrong, but I think that the rear lsd is sealed therefore additives aren't needed. I just put the NS in the rear diff too. Was that wrong??

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    Moderating on the run! Big Sky WRX's Avatar
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    Quote Originally Posted by zoinks
    I put in the NS back in the spring, and ever since then 1st and reverse grind worse than before. It's not all the time but definitely more than it did. I may try adding a bit of syncromesh to the mix to see if it helps. Also, I could be wrong, but I think that the rear lsd is sealed therefore additives aren't needed. I just put the NS in the rear diff too. Was that wrong??
    I don't think this is too uncommon- it was the reason I decided on the shockproof- I've yet to hear of anyone complain about shifting problems.

    I ran shockproof for close to 40,000.

    Big Sky

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