In changing the oil on my '03 WRX both times I've noticed to get the car to the proper reading on the dip stick you need to add more than the 4.8 qts. stated in the manual. It seems like I have to add more like a full five qts.
I have used an AMSOil SDF-20 (WRX application) Oil Filter which is a slightly different shape than the OE filter. I have, of course, idled the car after the change, turned the car off and allowed time for the oil to flow back to the pan.
I have found this same issue on other cars I own so I'm not that alarmed by the discrepency but wondered about other's experiences with the WRX.