i had a k&n make my MAF filthy as well. Only had it on for a couple thousands miles. Of course, it may not be known as a common problem because it wasnt enough to really change the way the car ran or idled, or through a code. But it was visibly dirty, and the car felt much more responsive after cleaning the MAF. I have since put a stock air box back in and a crappy paper filter. I noticed zero difference in butt dyno power with the K&N and MAF adaptor compared to stock airbox and paper filter