Running rich

I need some help please, My service engine light came on over the weekend. I took my car by the BMW Dealership and got the codes checked and the light reset. The fella in the service dept told me that my car is running rich in bank 1 and bank 2. I do have a k/n airfilter in my car do you think this could have anything to do with my car running rich ? This is the only thing I can think of. If its not the airfilter what do you think it could be. I drove 7 miles back to work and the light did not come back on. Any advice would be greatly appreciated. Thank's. My car is a 2004 Z4 3.0.

Oiled K&N filter ?

Over oiling can affect the MAF. As the MAF reads the airflow and hence tells the ECU how much fuel to add, it would be easy to get a rich mixture.