No, it doesn't have
to be Subaru oem coolant, but that's what I used.
If you do it the way I did it, then you'll only need one gallon of undiluted
Subaru OEM coolant which can be found either at the dealership or online, but you will end up diluting it about 50-50 with distilled
water (very important!!!) along with a single small bottle of Subaru OEM coolant conditioner and one bottle of Redline Waterwetter or something similar if you choose. When I replaced my coolant at 30k, I emptied the stock coolant, filled it with only distilled water, turned the car on and let it idle for a few mins, shut it off, drained the distilled water, refilled again with distilled water only, let it idle, drained that, and repeated one or two more times before I finally filled it with 50% coolant and 50% distilled water + the coolant conditioner + waterwetter. I did this because I don't have a machine to actually flush out the entire system, so I just repeated that process until the stuff that drained out was mostly clear and I felt pretty confident that I flushed it out as best I could. I then burped the system many times until I saw almost zero bubbles popping up and then I burped it once more for good measure. I had no problems with overheating or anything else since then and I'm almost at 60k and about to do the same thing again. The draining part makes a huge mess because the coolant/distilled water spills all over the frame and just gushes everywhere - even if you have a decently large sized catch-container, so have plenty of rags ready
Make sure everything is sufficiently cooled down before you go loosening any caps or you'll get burned pretty bad. It's a fun job, but rewarding knowing that you now have the knowledge to repeat the process in another 30k miles plus you'll know you saved some money on labor and you'll be happy that your car has some fresh fluid in it and that part of it will again be in tip-top shape