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#2563371 - 03/10/12 02:46 PM Why different grades for Subie front/rear diffs?
fdcg27 Offline


Registered: 09/25/09
Posts: 10924
Loc: OH
I need to change the diff fluid in both of the Subarus.
I can get a good price on M1 75W-90.
Subaru specifically recommends a 75W-90 for the rear diff, but an 80W-90 for the front one.
Does anyone have any idea why different grades of GL-5 would be recommended between the two?
Does anyone think that a 75W-90 would increase wear for the front diff if used in place of the recommended 80W-90?
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#2563388 - 03/10/12 02:59 PM Re: Why different grades for Subie front/rear diffs? [Re: fdcg27]
Bill in Utah Offline



Registered: 11/11/02
Posts: 12794
Loc: UT
Does not the front one also have to deal with the transfer of front to rear power along with the normal drive of front wheels?

As well as the Subaru works and how complex (read COST to fix) I'd follow their recommendations to the end of the earth...

Bill

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#2563444 - 03/10/12 03:59 PM Re: Why different grades for Subie front/rear diffs? [Re: fdcg27]
Trav Offline


Registered: 11/20/06
Posts: 12210
Loc: MA, Mittelfranken.de
75w-90 in automatic cars is fine front and rear, i am running in in mine without any problems. On manual units the front shares the diff oil so the need the 80w-90.
Something about the additives and syncro rings.

Check the Subie forums out, that's where i found this info when i was doing mine and was in the same boat.

http://www.subaruoutback.org/forums/99-d...smission-6.html
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#2563447 - 03/10/12 04:01 PM Re: Why different grades for Subie front/rear diffs? [Re: Trav]
Rand Offline


Registered: 08/20/03
Posts: 8953
Loc: Barberton,Ohio
you can use 75w90 in the front diff/MT just fine.. just make sure its not friction modified.

something like supertech 75w90 syn blend gl-5 is ideal

its semi-hard to find a non FM GL-5 75w90.. but if you do find it.. its an upgrade over 80w90 esp for winter shifting.

I currently have redline 75w90NS in mine.. I am trying to supertech next.

I tried the sub extra-s 75w90 that comes in the STI.. but it was too slippery and had bad shifting.. so out it went.


Edited by Rand (03/10/12 04:03 PM)
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#2563449 - 03/10/12 04:02 PM Re: Why different grades for Subie front/rear diffs? [Re: fdcg27]
Hollow Offline


Registered: 02/29/08
Posts: 1134
Loc: the South
Are they manual or automatics?

The manual shares fluid with the MT and front diff. Mobil 1 is not recommended for the MT, as the chemistry does not play nice with the metal used in the Subaru.

It is fine however with the front diff of the automatic, as they are separate units.

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#2563454 - 03/10/12 04:05 PM Re: Why different grades for Subie front/rear diffs? [Re: Hollow]
Rand Offline


Registered: 08/20/03
Posts: 8953
Loc: Barberton,Ohio
Originally Posted By: HollowEyes
Are they manual or automatics?

The manual shares fluid with the MT and front diff. Mobil 1 is not recommended for the MT, as the chemistry does not play nice with the metal used in the Subaru.

It is fine however with the front diff of the automatic, as they are separate units.


it is usually the friction modifiers that are wacky. you want FM in a diff.. and dont want any in a MT because they need friction for the shifting to be smooth etc.


the front diff on a MT needs several things which are in opposition.

GL-5 for hypoid gear protection (bad for MT)
FM for fuel economy (cant have with MT)

MT wants no FM's and GL-4 additive levels

so you have to live with GL-5 to protect the gears
and no FM so the MT shifts right.

Not sure who thought it was a good idea to have a combo unit but it think its not good engineering.

also if the front hypoid style diff gears go bad.. due to running gl-4 etc, you pretty much have to replace the whole transmission as just replacing the gears is REALLY expensive.

If you can find a gl-5 with the wet clutch jaso-mt? spec thats usually a step in the right direction for these as it means little or no FM's


Edited by Rand (03/10/12 04:08 PM)
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#2563814 - 03/10/12 09:50 PM Re: Why different grades for Subie front/rear diffs? [Re: fdcg27]
tdi-rick Offline


Registered: 02/02/04
Posts: 1827
Loc: the antipodes
Motul Gear 300 in the front.

Other really good fluids are Neo 75W-90HD and (Australian) Castrol Syntrax 75W-90.
Unfortunately Castrol OS names are all over the place and I don't know what the equivalent is.

All made for transaxle use and shift very, very well with good hypoid diff protection.
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