More than likely you have debris on the MAF sensing wire, whether it is from lack of a filter, excess oil from the K&N or both. Remove the MAF and rinse it with some carb cleaner - you'll see a small, delicate wire inside. Be careful not to damage it. Allow the cleaner to completely dry before turning the key on.
Filters that use oil on them have been forbidden from warranty coverage by almost every manufacturer. The oil can contaminate the sensing wire and cause driveability issues. I don't know of any OEM that condones cleaning the MAF but it can be done if care and good sense are used. The MAF is highly important to the operation of the car and anything you can do to keep it good condition is recommended. Personally, and I know there are many that think it's worth it, I can't see why anyone would think changing the air filter is such a great idea. Even with the most stretched imagination, what would a .3 second increase be worth to the average Joe, vs. reliability and mpg? It isn't going to turn you into a national hero or a sex god...
