The external filter was phased out around that year. Look on the side of the AT, there will wither be a spin on filter or a blank plate where one was in model years past. Occasionally they may be tucked behind the battery, but that is mostly on the H6 model.
OEM filter only if you do have one to replace.
Your car came with Subaru ATF HP...which there is very little aftermarket support for. Royal Purple and Amsoil both list the old Subaru ATF (Dex III) and newer ATF HP on their bottles as "meets or exceeds" specs.
If the price is reasonable, I'd stick with the OEM fluid. The Dex III types don't work well in the ATF HP applications.
I think when I got the trans filter for my car (2005) they said that subaru went to all internal in 2007, but it should be easy to find. The do it yourself guide on here is a great tool for AT fluid change. Good luck checking fluid levels though in the AT, mine sucks to read.
ok well thanks for the info,i stopped by NAPA and they said the book calls for a spin off type ,so i picked one up now before i go for it i need to get the fluid and then crawl under and take a look see.........