I just use a flat one (non-crushable, just an ordinary steel washer). Thomason Subaru, from whom I mail-order my parts, say they use the non-crushable kind in their service dept and it works fine. They tape one to the box of every oil-filter I buy, although there's no reason they shouldn't be re-usable.
I simply torque the drain-plug to 40 ft-lbs (the Haynes manual says
35-45) and I've never had any problem with stripped threads, the drain-plug coming loose, or leakage around the washer.
John
P.S. Once, I got my oil changed at some fast place in Flagstaff AZ. Later I looked and it seemed they hadn't put a washer in AT ALL. But I couldn't tell for sure. I took it back and they looked and said "yeah, there's a washer in there". Sure enough, when I got home and changed the oil again, there was no washer. Bastards ! Wish I could remember the name of the place. But there were no problems.