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I've attached all the procedures from the service manual for properly filling the CVT and both differentials. A normal "run of the mill" scan tool will not be able to monitor the transmission temperature. I bought an Innova 5510 diagnostic scan tool to monitor the CVT temp while performing the second part of the fill procedure. This specific scan tool works and has a long enough cord to monitor the temp under the car during the second part of the fill procedure. Probably not the cheapest option, but that's what I used.

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Basically the procedure goes like this:
1. Drain fluid cold to get as much out as possible.
2. Fill transmission until fluid overflows out of fill hole and temporarily install the fill plug (only finger tight).
3. Start car and hook up scan tool in order to monitor transmission temp.
4. Once the scan tool reads 35° Celsius, shift through the gears pausing in each gear 10 seconds (P-R-N-D-N-R-P).
5. Quickly go back under car with scan tool and fill transmission again through same fill hole until it once again overflows.
6. Reinstall fill plug and torque to spec.

You only have about a 6 minute window to perform steps 4 - 6. After that, the transmission temp will climb higher than the specified temperature range (35° - 45° Celsius).

In order to make this procedure as painless as possible, I bought two of these (one for CVT fluid, one for diff fluid):

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I used the right angle adapter that you can see in the picture at the link above. I also never pumped it up past 15 psi, even though it says it goes to 20 psi. You have to fill it with a funnel because the whole top doesn't screw off, just the orange cap that contains the pressure relief valve.

Hope this helps. Also, I'm using Amsoil CVT fluid in my car with no issues so far. I've done 3 drain and refills in order to get the majority of the oem fluid out. The drain and fill procedure outlined in this post only replaces about 40 - 45 percent of the fluid according to my own calculations.

 

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in CVT - Adjustment.pdf it says to "use new gasket"

What's the part number for this gasket?

I believe the part number is 803918060, although I'm not 100% sure on that. I've re-used my original gasket each time I've done a drain and fill on mine.

2018 Subaru Impreza 2.0L CVT LIMITED w/EyeSight WAGON Cover Differential Carrier. Cap Breather. Gasket 18X24X1.0. Differential - 803918060 - Genuine Subaru Part

Cover Differential Carrier. Cap Breather. Gasket 18X24X1.0. Differential Cover. M / #110411. Plug Complete Oil Drain. 2018 Subaru Impreza 2.0L CVT LIMITED w/EyeSight WAGON. Genuine Subaru Part - 803918060 (38315AA011, 38315AA030, 38315KA100)

parts.subaru.com

AT, TRANSMISSION CASE. 2018 Subaru Impreza 2.0L CVT LIMITED w/EyeSight WAGON | Subaru Parts Online

2018 Subaru Impreza 2.0L CVT LIMITED w/EyeSight WAGON AT, TRANSMISSION CASE TRANSMISSION CASE PARTS

parts.subaru.com