The K&N filters don't filter that well. They pass many small particles that would be trapped in a "paper" filter. Another problem is that the K&N filter, being a "cloth" design, needs to be oiled to work properly. The oil leaves a film in the intake system that accumulates upon the MAF sensor elements, thus contaminating the MAF quickly.
K&N filters are good where large volumes of air flow is required, but they are not good at filtering smaller particles. I've been running them on several of my personal vehicles and have noticed several things;
* engine oil gets darker quicker
* MAF gets contaminated and requires frequent cleaning
* performance is not perceptibly better than with paper filter
* MPG hss not changed to any noticable extent
Many people swear by the K&N filters, but I swear and cuss at them.
