Brad might have much more insight on this, but my first thought is to identify where the oil is coming from. Is it leaking above the heads into the intake area and getting pulled into the intake via a vacuum leak? I don't know if your engine is just refreshed and clean, or untouched for an unknown period of time. 928s do commonly have oil leaks that get into the valley between the heads, so perhaps this is making it into the intake and you are seeing it out of the air filter housing.
So, I would start by cleaning the area, if not already clean to determine if there is any leak. If none, then how oil gets to the air filter from inside the motor would be the next question I would ask.
928s, and perhaps more particularly the early models, would froth oil in the pan a bit and it would get piped up to the intake to be burned with the fuel mixture. If you are thrashing your 928 hard or you are running too much oil this could be a possibility as well. So review the oil fill and see if there is a hose to the intake from that area. If there is perhaps check to see if oil has been passing through it. There are many options, some cheap and ways to get by, and others more expensive, once you know.
I hope this helps a bit.
