The OEM filter has a little oil in it IIRC. To see if you have oil in the intake section, just take your IC off and see if there's a film of grease on the inside. Most stock WRX's will exhibit this and it is from oil blowby. It's not enough to harm anything and if it really bothers you, you can get a oil catch can. I put one on since I'm far from stock, but it's not a "gotta have it thing".