K&N filter question...Possible disturbing news!!!!

[Morgstang]

My car was idling erractic and took it to the Ford dealer today. They found that the Valve assembly on the throttle body wasnt functioning properly due to excess dirt buildup. When i asked the mechanic how it could have got that way he told me that it was probably from the K&N filter i have on my WMS Racing cold air kit. He said that they arent very good at filtering the small particles as well as normal filters. Also he said that the mass air sensor could be the next victim because of the filter because of the oil in the K&N getting trapped on the MAF sensors. This may not be worded exactly as he spoke it but i tried to get close. Question is if this is true do i go back to the stock airbox and filter OR is there another performance filter that filters better than the K&N. My car has 28K miles on it and is never drivin in dusty conditions. Thanks for your answers.