I recently shared this info with another user inquiring about their 2018 forester with a TR580 CVT :
“Your transmission holds 12qts of CVTF. When you drain only about 6qts will come out. Flushing is not recommended as it can break free particulates and debris that clog actuators and valve bodies and cause a variety of issues. A repeated drain and fill can be done to ensure that more than half the total fluid content has been replaced. Otherwise, a normal drain and refill will provide half new and half used fluid which is very acceptable if doing 30k mile intervals.
Your Subaru TR580 is the same transmission I have in my Crosstrek and same as is in the Imprezas as well.
The Subaru CVTs are very particular about fluid. Yes, generic high quality CVT fluid will allow them to function, but it is not ideal nor properly spec’d for these transmissions. It has been proven that they run and last best when filled with OEM fluid. For your model year I believe Subaru CVT-F II is called for, it is made by Idemitsu, the premier oil and lubricant manufacturer in Japan.
The Subaru bottled part number is:
SOA427V1660
The Idemitsu bottled and identical (and Subaru approved) fluid is the Idemitsu SB2 and it is available on Amazon much cheaper than the Subaru bottled variant.
https://a.co/d/3xneyab
Please be sure your mechanic is familiar with the proper procedure for TR580 fluid drain and refill! It must be done following some precise steps to ensure proper level at a certain temperature so that cavitation of the system does not occur. I would highly suggest having a dealership do this, maybe shop dealerships for the best price. My dealership does a CVT drain and fill for $238 which is a very very good price.”