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Old 09-22-2019, 03:04 AM   #1
Scooby Specialist
Member#: 99707
Join Date: Nov 2005
Chapter/Region: MWSOC
Location: Missouri
2018 WRX Premium CWP

Default 30k Front diff / trans + rear diff oil change

So I finally completed this today on my 2018 wrx. It went way better and easier than I expected it would.

Materials I gathered were as follows.

1. 5qt of subaru 75w-90 (ended up needing 4)
2. 1 gasket/washer part #803926090
3. a torx T70
4. a pump for the rear diff (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1)
5. a funnel to reach the dipstick hole on the transmission (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1)
6. a bucket to measure what i actually managed to drain from the transmission / rear diff (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1)

To start I jacked the car up and had it supported at all 4 jack points by jack stands evenly.


once that was done I thought I should tackle the front transmission / diff housing as it i imagine would be the most problematic.

yanked the dipstick out and inserted previous funnel into the hole. it sits up top sandwiched by the intercooler and wiring harness just fine.

went below and cracked the drain bolt to the transmission loose.

it came loose rather easily. proceeded to chuck my bucket underneath and try not to drench myself in only what i can describe as satan's jizz. trans fluid has a special level of "ohgodwhy" going for it.

managed to make not too much of a disaster mess once i get the bolt free and clear. seriously i kind of wish i had gone slower with this, but the fluid especially warm really flows out of the bottom at a rapid pace.

I let this drain a while after the inital MASS DUMP. once it slowed down to a rather sedate.... tick.... tick... tick... pace. put the plug and new gasket back in. hand tightened. ill go over final tight later.

with the drain plug on the transmission just snugged up so it doesnt leak. proceeded to add 3qt of the fluid i bought. I myself only added 3qt. Thats what my bucket said i removed. so thats what i put back in. the front holds 3.5qt total and im assuming i cant get it all out or the factory underfilled.

either way I added 3 which my bucket says was what i removed. I later checked the dipstick and it was spot on as far as level so im happy.

= what it needs to have.... ok neat.

moving on to the rear.


im not the strongest dude ever and the rear bolts on the diff gave me some trouble. not a ton, but they were quite stubborn. also note i couldn't get a rather compact electric impact in there so this needed just straight muscl of which i am not blessed with a significant amount of.

so i decided to use the left rear wheel to brace my feet and yank on the ratchet/socket? whatever the **** its called... the rear needs a 1/2" drive .... thing. work that shaft like itll bring daddy back. The fill bolt should now be free.

now attempt to crack the drain bolt loose.

these both require some serious effort.

get em both loose and then clean off the plugs.

reinsert the drain plug (lower one with magnet).

use pump and pump .8qt back into rear diff. pretty much right as it starts leaking out the top fill hole is good.

transmission drain bolt needs 51lb/ft of torque im not sure on the rear, but i used the gudentite method of if i tightened it as hard as i could without using the bracing technique i needed to do to loosen them im probably ok.

post fill and swap drive test. i always test what ive just done to make sure nothing is super off / wrong.

notes upon doing this and driving make the 1.5miles to the gas station for a soda.

shifting was much smoother. overall shifting felt improved.

i suspect the front diff/trans takes a beating thermally just due to its location.

overall even at only 32k miles when I did this.... I would highly recommend doing this service and not just (inspect) as the manual calls it. change this fluid out asap... you wont regret it.
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Old 09-22-2019, 11:27 AM   #2
Scooby Newbie
Member#: 477489
Join Date: Nov 2017
2018 WRX Limited


I did the same service last weekend on my 2018 WRX, at 20k miles. I used Motul 75W90, and shifting has improved a bit. Fluid from the front looked dirty and black, while the rear seemed to be not so bad.
The manual say 3.5/0.8 qts front/rear, but mine was full with just a bit under that amount.
Overall,the process wasn't too difficult. The rear is a bit of a pain to break loose, but a 1/2 inch breaker bar helps. One thing that tripped me a bit was the temperature sensor that was mentioned in the service manual and elsewhere, but apparently this is a thing for the STI and not the WRX, so one less thing to worry about.
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Old 04-24-2020, 05:55 PM   #3
Scooby Newbie
Member#: 514028
Join Date: Apr 2020
Location: Welland, ON
2010 Impreza WRX


that pump for the rear diff looks really handy, thanks for the link
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