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Old 02-27-2004, 10:05 PM   #1
Maniac
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Question Manual Transmission Gear Oil Viscosity/Grade Question...

Hi, all!

I am getting ready to change my diff and tranny oils (2002 WRX with 33Kmi).

I decided to go with Mobil 1 75W90 in the rear diff and Valvoline (regular Dino used in the box from the factory) oil in the tranny / front diff. I heard that synthetics make the synchros too slippery if used in the tranny.

The only problem is, my local Pep Girls only carries 80W90 oils... Is it OK to use these, are they better in hot weather (100+) than the 75W90? Should I try to get the 75W90 instead of 80W90, as specified in the manual...

Thanks for the input!
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Old 02-27-2004, 11:55 PM   #2
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Ya, 08w90 is ok... it was talked about in a thread about a week back...
I wouldnt put mobil 1 in the rear diff, put in what ever you are putting in the tranny.
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Old 02-28-2004, 01:43 AM   #3
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Quote:
Originally posted by totoherbs
...
I wouldnt put mobil 1 in the rear diff, put in what ever you are putting in the tranny.
Really? Why? I thought synth is good for gears...
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Old 03-09-2004, 08:26 PM   #4
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Mobil 1 75W90 is fine for the diff. It's the correct weight, meets the proper spec (GL-5) and it's a synthetic. You can't get much more appropriate for the diff than that.
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Old 03-10-2004, 02:07 AM   #5
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Quote:
Originally posted by Maniac
Really? Why? I thought synth is good for gears...
It just seems stupid to put different stuff in the diff then in the tranny...
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Old 03-10-2004, 04:13 AM   #6
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I thought the lower the friction the better it is for gears, thus synthetic in the rear diff for longer life...

In the tranny, however, the synchros need to rub in order to mesh gears. Too slippery is bad for synchros, thus Dino oil in the transmission...

That's my logic...
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Old 03-10-2004, 07:50 AM   #7
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Quote:
Originally posted by totoherbs
It just seems stupid to put different stuff in the diff then in the tranny...
Why is that exactly? You're obviously not thinking very clearly about this:

The diff is a sealed type. It doesn't share any of its fluid with any other mechanism. You can use whatever you want in the diff that meets Subby specs, and Mobil 1 is a very good choice.

You probably don't want Mobil 1 in the tranny, since despite the fact that it meets Subby specs for tranny fluid, most people have had bad results with it (too thin for the synchros).

So running different tranny and diff oils is hardly "stupid", as you so eloquently put it.
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Old 03-10-2004, 09:24 AM   #8
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Quote:
Originally posted by heffergm
The diff is a sealed type. It doesn't share any of its fluid with any other mechanism.

This isn't true in the STI as it is a mechanical LSD rather than a sealed Viscous unit.
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Old 03-10-2004, 04:04 PM   #9
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Quote:
Originally posted by heffergm
Why is that exactly? You're obviously not thinking very clearly about this:

The diff is a sealed type. It doesn't share any of its fluid with any other mechanism. You can use whatever you want in the diff that meets Subby specs, and Mobil 1 is a very good choice.

You probably don't want Mobil 1 in the tranny, since despite the fact that it meets Subby specs for tranny fluid, most people have had bad results with it (too thin for the synchros).

So running different tranny and diff oils is hardly "stupid", as you so eloquently put it.
Moble one does not belong in the tranny but as for other synths it a different story...

Well as I see it; its not like you will see any benifit from it, so why? And wouldnt the "sliperyness" of the moble one take longer to heat up and grap in the rear diff? Just put what ever you have got going in the tranny in the rear end and save yourself some money... If you realy were worried you would put in a high grade fluid like redline, neo, or amsoil.

BTW ive got NEO in both, and have no problems.
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Old 03-10-2004, 06:10 PM   #10
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80w90 should be ok because its not cold in so cal.

post your results after u fishish . i plan to do the same thing on my car.
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Old 03-11-2004, 01:29 AM   #11
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I wouldn't use 80w90 for the WRX, because the manual doesn't specify it. If anything goes wrong with your tranny, not using the right type of oil doesn't really look good.

Also, it can be fine to use the same oil in the tranny and diff, or it could be a problem. It depends on the oil you use. Some are versatile enough to be used in both, some oils should be only for one or the other. http://www.cobbtuning.com/tech/gearoil/index.html
This should help out a little bit.

For the record, I put Redline 75w90NS in both, and I don't really care for it. I'm guessing it's doing fine in the diff, but the synchros really take a long time to come up to speed. Possibly I'll try MT-90 next time, or a different brand. We'll see.
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Old 03-11-2004, 01:37 AM   #12
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MT-90 is not a gl-5 and dispite what it says on redlines site about gl-5 being bad for the tranny life. My guess is on subaru is the safer bet.... over a company trying to pimp its own oil.
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Old 03-11-2004, 08:05 AM   #13
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Quote:
This isn't true in the STI as it is a mechanical LSD rather than a sealed Viscous unit.
We're not talking about the STi.

Quote:
I wouldn't use 80w90 for the WRX, because the manual doesn't specify it.
You need to go re-read your manual. It specifies various viscosities and ranges for varying climates. 80W90 is perfectly fine, especially if you live in So. Cal where it's never going to get anywhere near freezing.

Quote:
Well as I see it; its not like you will see any benifit from it, so why? And wouldnt the "sliperyness" of the moble one take longer to heat up and grap in the rear diff?
Your questions show you have absolutely no idea what you're talking about, so I'm not going to waste anymore effort trying to explain. I'd suggest doing some reading on how a differential actually works. If, at that point, you're still confused, I give up.
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Old 03-11-2004, 08:12 AM   #14
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Quote:
Originally posted by heffergm


Your questions show you have absolutely no idea what you're talking about, so I'm not going to waste anymore effort trying to explain. I'd suggest doing some reading on how a differential actually works. If, at that point, you're still confused, I give up.


The viscous limited-slip rear differential uses fluid to apply torque to both wheels; if a wheel starts to spin, it causes half of the housing holding the fluid to spin. The spinning fluid gets hot and thinker alowing the transfer from one half to the other. Depending on the fluid it can take place quickly or not so quickly.
The housing in Viscous limited slip differentials is, as has been said already a sealed unit. Because of this it can use regular gear oil for rwd apps and transaxle oil for front wheel drive apps.

The STi uses a clutch type mechanical rear diff.

Last edited by totoherbs; 03-11-2004 at 08:27 AM.
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Old 03-11-2004, 09:02 AM   #15
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Quote:
Originally posted by heffergm
We're not talking about the STi.
If somebody with an STI happened to open this thread they might find it useful to know that it doesn't pertain to them...
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Old 03-11-2004, 09:25 AM   #16
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Quote:
Originally posted by MooreWRX
If somebody with an STI happened to open this thread they might find it useful to know that it doesn't pertain to them...

The originally poster owns a WRX... I fail to see how the STi came up at all.
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Old 03-11-2004, 11:47 AM   #17
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Quote:
The spinning fluid gets hot and thinker alowing the transfer from one half to the other. Depending on the fluid it can take place quickly or not so quickly.
The gear oil you put in the rear diff is not the fluid that makes the viscous LSD work. I think the heating of the LSD fluid is independent of whatever oil you put in the diff.
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Old 03-12-2004, 06:39 AM   #18
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Quote:
Originally posted by leecea
The gear oil you put in the rear diff is not the fluid that makes the viscous LSD work. I think the heating of the LSD fluid is independent of whatever oil you put in the diff.
Realy? hmmmm ill look for info on it....
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Old 03-13-2004, 06:10 AM   #19
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Exactly: that's what "sealed" means

Both the rear and center diffs - WRX - are sealed viscous LSDs, with the special fluid completely contained within them.

The oil you put in the trans and rear diff lubricate all the other parts - shafts, gears, synchros, bearing...but has no bearing (pun intended) on the operation of the LSDs.
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Old 03-13-2004, 08:22 AM   #20
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Quote:
Originally posted by GravelRash
Exactly: that's what "sealed" means

Both the rear and center diffs - WRX - are sealed viscous LSDs, with the special fluid completely contained within them.

The oil you put in the trans and rear diff lubricate all the other parts - shafts, gears, synchros, bearing...but has no bearing (pun intended) on the operation of the LSDs.
OO... I see thanks...
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Old 03-17-2004, 05:57 PM   #21
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i had 80w90 in my trans and rear diff afer i changes the first time.. in the cold am of NJ it felt like muck when shifiting... so i went back to 75w90 mobil 1 in both.. no problems since.. only diff is i can tell mobil1 is way better in the cold am... and two i cant downshift into first as easy but its ok.. dont do it that often...
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