I've never been a fan of K&N air filters. In order to let more air through you have reduce the restrictions. You can either increase the surface area (more pores) or increase the pore size. Since there's a finite amount of space to pack more pleats, they have to increase pore size. This lets more dirt through.
Also, the washable lifetime filters are more prone to user error. Oil them too much when cleaning and you can end up fouling the MAF.
K&N Air Filter Review - Debunking the Myths (and why OEM is better)
Air Filtration Test
Marketing is a powerful thing. I've heard so many people say "K&N filters are supposed to be the best" but have no idea where that information came from, and have seen zero evidence to substantiate that claim. Most of the time they aren't even sure what they are supposed to be "the best" at.
I've seen many tests like the ones you posted, and they all prove the same thing over and over again. K&N filters (not picking on K&N specifically... other filters of that type too) do not filter as well as standard OEM filters, which means more particulates in your engine.
It is true that airflow can be marginally better with these filters, but I think the majority of the public fails to understand that this is completely irrelevant in the real world. Yes the POTENTIAL is there for greater airflow, but the engine is already getting all the air it needs, so there is nothing to be gained. If your OEM filter prevented the engine from receiving the proper amount of air, then sure, a less restrictive filter would help and may even produce more horsepower. But that simply isn't the case. Can you really believe that an engine manufacturer would design an intake system and not bother to work out how much airflow the engine required? The fact is, the intake system of any car is designed to deliver as much air as the engine needs, with plenty of buffer to allow for dirty filters, bugs, etc. The engine pulls in only what it needs, so putting on a less restrictive filter will not cause the engine to suddenly use more air, and thus it is simply not possible to increase horsepower that way.
How about this: If you replace your typical 1/4" fuel lines with a garden hose, you'll get more horsepower... more gas = more horsepower right? Well, no. The engine is already using all the fuel it needs, so increasing the POTENTIAL to deliver more fuel is pointless.
It boils down to this: standard OEM type air filters filter better and will keep your engine cleaner than a K&N, plus they are perfectly capable of supplying all the airflow your car needs. K&N filters allow more particulates to enter your engine, they commonly cause problems with oil deposits on MAF sensors, and do nothing to increase power. To me it's a no-brainer.
