Any fluid equivalent to Subaru ATF-HP is what you should use. The capacity of the transmission is 10.4 quarts. if you just pull the drain plug you'll only get about 4 quarts out. So if you do 3-4 of those you'll have mostly fresh fluid. If you decide to do a flush by pulling the lines off the radiator have a friend help you determine which hoses are in/out. Put the in hose in a bucket of fresh fluid and wait for the out hose to run the same color as the new fluid. only idle while you do this, it takes less than 5 minutes. Near the end of this process it is a good idea to shift it through the gears (R, D, 2, 1 if applicable) so fresh fluid makes it through all the valve body channels.