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Change out your rear differential fluid now!

81K views 54 replies 40 participants last post by  DockingPilot 
#1 · (Edited)
After only 23,000 miles I had lots of shavings and a square chunk of metal the size of a BB clustered around the magnet. The dino oil looked trashed too. I know I probably wouldn't have had this much wear and tear if I had switched to Motul 300 synthetic in the beginning. :icon_cry:

So what causes heavy wear in the rear differntial? It's not like I am enaging it manually like the transmission....replaced that with Motul 300 too because it it looked like tea. I'm thinking 20K intervals should be about right for the rear differential but I'm thinking 30K for the transmission. Shifts like a drream now! :biggrin:
 
#8 ·
Update--I just changed my rear differential fluid again. It had only been in for 8,000 miles(31K total) and this time there was just a little bit of black muck on the magnet(probably leftovers coating the housing from before) and the oil itself looked like it just came out of the bottle. I'll give Motul all the credit for this happy ending. :Banane35:
 
#9 ·
I'll give Motul all the credit for this happy ending. :Banane35:
I doubt that that has anything to do with it. My initial change was very clean and my second change with Mobil 1 was clear as a bell. You just had a lotof crap in there from the factory and it discolored the oil.

But the good news is that things look good and you have no noise (I assume). Certainly not knocking Motul.its as good as it gets.
 
#12 ·
I just changed the differential oils on my 2008 AT with 13,000 miles. I had collected various posts on how to do it, and noted these items of interest:

1. It was not particularly hard to break loose the rear diff plugs. They don't have gaskets, but wedge themselves, so I put a little Hylomar blue gel sealant on the threads. It does not get hard, but will stop seepage.

2. The front diff plug uses a hard thick solid copper gasket. I had bought a $3 new one, but the old one was perfect and could have been reused.

3. Although both diffs use the same gear oil, the rear oil was watery and gray, while the front oil was still light amber and viscous. The rear plug magnet also had a thick coating of black ferrous fuzz, while the front magnet had only a light coating.

4. I replaced the OEM dino oil with Redline synthetic 75W90 GL5 gear oil. This has got to be a "must" for the rear differential, which quickly destroys dino oil. I also wish I had done this much sooner. Subaru really ought to require this as part of the first 7500-mile service.

5. The previous posts said buy 3 quarts because the rear diff takes almost 1 quart and the front diff takes 1.3 quarts. With the car level, I stopped pumping the rear diff as soon as it overflowed, and put the rest of that quart plus another quart into the front diff. The front diff came exactly to the F (full) mark. So 2 quarts fill both differentials if the car is level. If the car is ramped, oil flows to the front of the long rear housing, which will be overfilled when the car is level, and also require buying a 3rd quart for the front differential, of which only a little bit will be used.
 
#45 ·
2. The front diff plug uses a hard thick solid copper gasket. I had bought a $3 new one, but the old one was perfect and could have been reused. Is it 1/2" gasket like the engine oil drain plug?

4. I replaced the OEM dino oil with Redline synthetic 75W90 GL5 gear oil. This has got to be a "must" for the rear differential, which quickly destroys dino oil. I also wish I had done this much sooner. Subaru really ought to require this as part of the first 7500-mile service. I've read good things about the Red Line 75W90 and pretty sure that is the route I'll go
This thread is pretty awesome. But mainly speaks to the MT crowd, I am about to change the fluids and want to make sure I have this right for my Auto's.

AT 4EAT:
Front Diff: 1.3 US Quarts gear oil
AT: 9.8-10.1 US Quarts but drain and fill is around 3.5-4 US quarts, dexIII ATF, try to stay away from synthetic?
Rear Diff: .8 US Quarts gear oil (same for either MT/AT?)
 
#16 ·
I think we need to start a photo gallery of transmission chunks... I am getting my AT fluid changed and Diffs also next week. At 120K we'll see what they look like.
Your 1999 with big external transmission filter should not have any transmission chunks. Wish my 2008 had that filter.
If your rear differential oil has never been changed, it could be scary. Black mud, with a big ball of spiky fuzz on the magnet.
 
#28 ·
After seeing this thread, I want to change my diffs and transmission oil. I have a 2005 XT 5-spd with 107k. I bought it with 69k and have no idea if the fluid was changed at all.

What oil should I use that is easy to find locally? I have an Amsoil dealer right down the road from me. Also how mant quarts will I need?

Im doing a timing belt next week, so why not do everything all at once and have a brand new car again, well almost.
 
#29 ·
I want to change my diffs and transmission oil. I have a 2005 XT 5-spd with 107k. I bought it with 69k and have no idea if the fluid was changed at all.

What oil should I use that is easy to find locally? I have an Amsoil dealer right down the road from me. Also how mant quarts will I need?
Most of this info in detailed in Sections 11 and 12 in your owner's manual.

You need ~3.7 U.S. qts of GL-5 75W-90 gear oil for the transaxle (combination manual trans plus front diff). Because it's used in the MT, this oil must be without limited slip (LS) additives. Synthetic gear oil is recommended; I use Mobil Delvac, but it can be hard to find.

Check the type/style of MT drain plug (hex or Torx?) before you begin work. Don't corn-fuze the engine oil drain plug and the MT fluid drain plug. The transaxle is filled through the MT dipstick tube.

You need ~0.8 U.S. qts of GL-5 75W-90 gear oil for the rear diff. Synthetic is recommended, with our without LS additives is OK. I have some leftover Subaru Extra-S from previous service, so I'll probably use that. Mobil 1 seems to be readily available and popular for this particular application.

The rear diff has two plugs. Loosen the upper fill plug before removing the lower drain plug. The correct tool is a 13mm square drive; some use a 1/2" (12.7mm) drive.

HTH,
Jim / crewzer
 
#31 · (Edited)
FML....I was thinking my MT only needed 1.2 quarts. Drained it only to find 2.8 quarts drained out. I'll have to read more carefully and confirm with my owners guide next time. Oh and yeah I changed out the rear diff fluid (17,256 miles) and yeah it was nasty with plenty of metal shavings stuck to the magnet. Btw has anyone used royal purple max gear in the rear diff/manual tranny?
 
#32 ·
I've used Royal Purple in both my rear diff and tranny. I would not recommend using it in the transmission. It caused nice whine and me to grind 4th gear every time I was at WOT. Replaced the Royal Purple with Grimmspeed's coocktail and problem solved transmission shifts like new.
 
#33 ·
Well being dumb and hard headed like i am (also not willing to tell wifey i spent 40 bucks on the wrong fluid) i ended up with my own half baked cocktail in the manual tranny. One quart factory fill plus 2.25 quarts RP 75w-90plus 1/2 quart lucas oil synthetic 75w-90. Drove 350 miles into the Houston area and so far it actually feels noticeably smoother. Even tore up the sand on the beach and shifted with ease. Guess i got lucky!
 
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