I also would recommend using standard o rings for this, the black buna n.
I rebuilt once using the fep, worked for awhile, perhaps never quite had full assist, but then the pump started leaking again onto the engine block.
Went down to rocket seals (they are local to me) and the guy there said that the buna n rings were more than sufficient for ATF. He also said he has recieved many orders and walk in customers rebuilding subaru p.s. pumps I believe as a direct result of this thread. I recommend Rocket Seals.
Thank you cadillac and this thread, saved me a bunch of cash.
But anyway I think the black rings are the way to go. My pump is now better than ever. Even if the buna n doesnt last quite as long, this rebuild is pretty simple especially if you have done it once or twice already. I got confused in a couple places but just zoomed in on cadillacs pics and was good to go.