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1998 Legacy Outback AWD 2.5L manual
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Discussion Starter · #1 ·
Hi all,
What are your recommendations on fluids for the rear differential? And also (I haven't looked yet), the front diff. if there is one.

I'm looking to empty and fill the rear diff at least.
 

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Given the age of the car, and probably the mileage, just about any good quality, 75W90 GL-5 gear oil will do fine. (That's what Subaru recommends.)

If you're in MI, and the car might be parked outside overnight in the winter, perhaps consider a synthetic gear oil (same 75W-90, GL-5 specs). Synthetics usually have somewhat better low temperature pour points, meaning they are somewhat more fluid, so on a cold start up they will circulate better than a regular gear oil. However, the difference only comes into play at temperatures well below 0 F.

On your manual transmission car, you have only a rear differential that can be serviced separately. There is a front differential, but it's part of the manual transmission, and shares the transmission's gear oil. The transmission can use the same gear oil as the rear differential with one exception I'm aware of (others here might have more about this): Mobil 1 gear oil, which is a highly regarded synthetic, is not recommended for use in the Subaru manual transmission. Consequently, if you decide to use the same gear oil in both the rear differential and the transmission, don't use Mobil 1.

There's very informative threads in the DIY sub-forum here about changing gear oil.

One definite recommendation: Remove the upper (fill) plug on the rear differential before removing the lower drain plug. The plugs often are very difficult to remove, especially if the gear oil hasn't been changed in a long time. You don't want to remove the lower one first (oil drains out) and then find the upper one is seized in place.
 

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1998 Legacy Outback AWD 2.5L manual
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Discussion Starter · #3 ·
that's great info! anything else I should know?
 

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Perhaps look at: http://www.subaruoutback.org/forums/111-gen-1-1995-1999/6390-rear-differential-oil-change-tool.html.

Rear differential:

Not sure about your 98, but I believe it too has the square fittings for the fill and drain plugs at the rear differential. If so, these are nominally 1/2-inch, but are actually 13 mm. A 1/2-drive works most of the time, but could be problematic if the plugs are seized. I picked up a specific 13 mm socket as I change the fluid regularly and don't want to develop cumulative wear due to the slightly underside 1/2-inch drive. (KTC 13mm Differential Socket AC301-13 - Ultimatetoolco.com)

Sometimes, a breaker bar lifted by a bottle or shop jack is used to help get the plugs moving out.

Don't overtighten when replacing the plugs. It could risk cracking the aluminum rear cover, and isn't necessary -- the pipe threads will tend to self seal. A bit of oil resistant thread sealer could be used, but many don't use anything without any seepage.

Most gear oil comes in quart bottles that aren't easy to use to fill the differential through the upper opening. Auto and sports shops sell small pumps that will screw down onto the bottles with a long outlet hose. That's what I use to fill the rear differential.

The proper level is to the bottom of the upper (fill) opening, with the car level.

Let us know how it works out . . .
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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Plain's post mirrors my 2-times performance of this maintenance item. I used my jack to bust the fill and drain plugs loose, and a pump from Amazon to fill.

Just wanna mention, the drain plug has a magnet on it. Be prepared to wipe off 1/4-1/2 teaspoon of sludge.

And the fluid stinks - try not to take any clothing or rags/paper towels into your house that have diff fluid on them.
 

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Any good brand name gear lube should work fine as long as its replaced once in a while...

I used Schaeffer this go around, but would probably use Mobil 1 next time because I use it in my truck too (but I have an auto with two separate differential boxes). But as noted previously, there are a lot of threads that indicate Mobil 1 and Subaru manual gear boxes don't work well together.
 

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1998 Legacy Outback AWD 2.5L manual
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Discussion Starter · #8 ·
good stuff guys! what's the fluid capacity?
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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check your manual but, the rear I THINK is about .8 qts. (or is it litres?)
 

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0.8 litres rear, 1.2 litres front. U.S. qts is pretty well the same at these volumes -- 0.845 and 1.268, respectively.

When the differentials are drained, there's usually a small amount left in the bottom below the lip of the drain plug. I usually pick up 2 litres/qts and it's enough, provided none is spilled.
 

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1998 Legacy Outback AWD 2.5L manual
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Discussion Starter · #11 ·
Yep, it was around .8 qts that I got out of it. :) It went well and easy enough. The bottom plug was much tighter than the top one so I needed to use a hand crank jack on my 1/2 ratchet. That worked and the rest was easy. I cleaned off the magnet on the bottom plug; it had a mild buildup of metal sludginess. It was difficult refilling the oil, but I was able to squeeze the bottle and squirt like .5 qt in, then I stuffed the bottle above the sway bar to feed the rest in.
 

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Hi all,
What are your recommendations on fluids for the rear differential? And also (I haven't looked yet), the front diff. if there is one.

I'm looking to empty and fill the rear diff at least.
I run nothing but amsoil fluids except I like mobile 1 engine oil.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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Jw - this thread is 9 years old.
 

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Thanks I didn't know 2012 was 9 years ago, guess I'll just keep to myself from now on lol. Not like anyone could stumble across a 9 year old post and look at the comments or anything.
Don't worry about it, I think most users actually use the search now often when they first join, and then bump old posts:)

Eventually we get lazy and don't use the search anymore.

Sent from my SM-G960U using Tapatalk
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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Thanks I didn't know 2012 was 9 years ago, guess I'll just keep to myself from now on lol. Not like anyone could stumble across a 9 year old post and look at the comments or anything.
no offense was intended, just a heads-up that there may not be a response from the OP (bigcee)

amsoil makes good products.
 
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