If you have the manual, it should tell how many quarts you need (Haynes repair manuals are nice to have around to, for little break-fix projects). Have you done any of the recommended maintenance? (rear diff changes, timing belt, etc). Keeping that stuff up will keep your car running longer.
On my wife's previous 95 outback (2.2L), the first owner had let the timing belt go 30k miles past recommended change time. We didn't know that, but every so often, the car wouldn't start, just turn over and over. 5 minutes later it would start fine. CEL would always come on, we finally got it checked out, and it was the crankshaft position sensor because IT HAD TINY PIECES OF TIMING BELT ON IT!
So yeah, getting those CELs checked and/or resolved is always good.