Originally Posted by wstar

The basic argument for K&N's approach is that the filter media they're using has huge holes in it. If you ran a K&N filter dry, it would get way better flow than anything else, at the cost of letting tons of dirt through. Their design idea is to use a way-too-breathable-for-safety filter media and then wet it with oil so that the dirt sticks to the oil instead of going into your engine. They really do flow pretty well, better than stock anyways. I can't comment on how they compare to other non-stock options like the Cosworth though.

And yes, there has been some long-running debate between K&N and their detractors about whether the K&N setup is bad for your engine, either because even with the oil it still lets too much dirt through, or whether the oil itself flows into your intake in very small concentrations, which builds up on your MAF/MAS sensors and causes them to degrade and read incorrectly.

K&N's side of the argument is that they do extensive SAE-backed testing of their product, and it filters to industry specs and does not introduce their oil onto your MAF, assuming you oil it properly (and blaming over-oiling for the MAF issues essentially). The detractors have had labs do tests that seem to contradict in some areas though. I honestly think most of the debate comes down to whether the K&N filter in question is perfectly maintained and has exactly the right amount of oil (too little = dirt gets through, too much = K&N oil on your MAF sensor).

In any case, lots of people use them and for the vast majority of people they seem to work fine.