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Fluid Quantity for Trans/Diff and Rear Diff?
Fluid Quantity for Trans/Diff and Rear Diff?
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Old 08-17-19, 03:51 PM   #1
rs191
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Default Fluid Quantity for Trans/Diff and Rear Diff?

I've read the threads and watched YouTube videos. There's one question I haven't seen answered, unless I missed it.

Do you guys actually measure 4.0 or 4.1 liters (4.3 qts.) into the trans/diff or do you pump gear oil into the fill hole until it overflows?

I know the rear differential is filled until it starts to overflow, which should be around 1 liter or 1.1 quarts. Do most people buy five quarts or liters, drain-and-fill the rear diff and then put the remaining gear oil into the front without measuring?

Several people have said they buy 5 liters of Motul Gear300 75W-90 and that is enough for both ends of the car. What's been your experience? I am about to order the Motul Gear300.

(I'm doing this on a 2015 STI with just over 30,000 miles.)
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Old 08-17-19, 04:56 PM   #2
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

Quote:
Originally Posted by rs191 View Post
I've read the threads and watched YouTube videos. There's one question I haven't seen answered, unless I missed it.

Do you guys actually measure 4.0 or 4.1 liters (4.3 qts.) into the trans/diff or do you pump gear oil into the fill hole until it overflows?

I know the rear differential is filled until it starts to overflow, which should be around 1 liter or 1.1 quarts. Do most people buy five quarts or liters, drain-and-fill the rear diff and then put the remaining gear oil into the front without measuring?

Several people have said they buy 5 liters of Motul Gear300 75W-90 and that is enough for both ends of the car. What's been your experience? I am about to order the Motul Gear300.

(I'm doing this on a 2015 STI with just over 30,000 miles.)

I just used about 5 quarts of motul gear 300, 4 quarts on the tranny and a quart on the rear diff.
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Old 08-17-19, 06:25 PM   #3
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

ajsti24,
I thought that was probably the way that most people did it. Thanks.

Been changing trans and diff fluids on an old Miata for many years, for track days, and that manual says "fill to bottom of the fill plug hole" for both the transmission and differential.
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Old 08-19-19, 11:30 AM   #4
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

I just pump it till it overflows. Last time I did the transmission, 4qt wasn't quite enough, but the 5th qt was only barely started when it hit the level. I can't help on the rear diff though, since I'm running an aftermarket diff cover.
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Old 08-19-19, 11:38 AM   #5
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

I'm about to do this myself for the first time. Thanks for the advice and the post. The comment regarding the 5th quart not being taken much makes sense. I believe the transmission is 4.3 qt.
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Old 08-20-19, 04:17 AM   #6
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

Motul is sold in liters, at least everything I've bought, and 5 liters is about 5.3 quarts. So I got 5 liters and had a little bit left over.
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Old 08-20-19, 04:25 AM   #7
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

I just fill both the trans and diff until they overflow with the subaru gear oil. I think its preference, i know alot of guys love the motul stuff and others swears by the oem gear oil. Either or is fine but i would make sure you have an extra bottle of whatever you decide to go with incase the job needs more fluid for whatever reason.
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Old 08-20-19, 06:51 AM   #8
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

+1 on 5 liters, rallysport sold me that many when i advised them what maintenance i was doing, have ~1/2 liter remaining after
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Old 08-20-19, 09:10 AM   #9
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

Appreciate the responses, thanks. I am now totally in the "fill front trans/diff to overflow" group, not the "use four bottles" group.

Some people didn't say "fill to overflow", which makes me think they could be putting only four Quarts (4 Liters for Motul) and not topping up the trans/diff like it should be.

I kept searching and found service manual pages on a site. From the 2015 WRX manual, which also has all STI-specific instructions, page "6MT(TY85)-26" says
"Pour transmission oil to the bottom end of the filler plug."
"NOTE: carefully refill transmission gear oil while checking the level. Excessive or insufficient oil must be avoided."
"Recommended oil: GL-5 (75W-90) or equivalent."
"Transmission gear oil capacity: 4.1 L (4.3 U.S. qt., 3.6 Imp. qt.)

Under "Inspection" it says "Check that the transmission gear oil level is up to the bottom of the filler plug. If the transmission gear oil level is low, confirm that there is no oil leakage and refill up to the bottom of filler plug."
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Old 08-21-19, 06:59 AM   #10
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Default Re: Fluid Quantity for Trans/Diff and Rear Diff?

I just bought 6 qts. of Eneos GL-5 75W-90 from amazon and paid about $60 total with free shipping.
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