I will say that the latest K&N I looked at looked denser than the old ones I have. Still, overoiling aside, in light of the better stuff that's out there and my improved knowledge base, I just can't get behind them any more. Let's look at it from three viewpoints. And since K&N isn't the only company offereing oiled cotton gauze (OCG)fitler, I'm just going to use that terminology rather than just putting K&N down.
Performance: No doubt the average OCG filter will flow more air than a stock filter. From here it depends on whether we are talking about a replacement element or a tuned intake system. With regards to a performance element in a stock intake system, bear in mind that whatever restrictions exist are most often with the plumbing, not the filter. Seldom do you see any gains with a replacement OCG and, if you do, they come at the upper rpm range where not many stock engines go and the people driving those stock engines are seldom inclined to go. The full systems do a bit better, because they eliminate the plumbing issues, but unless the stock intake system is pretty poorly designed (not seen very often these days), the gains are still pretty modest and, again, mostly at the upper rpm range. Sometimes the way the new performance system is made offers some gains in the midrange. If you put an intake system on a modified engine where the airflow needs were increased by EFI tuning, major exhaust mods, cams, etc, and it was having a hard time pulling through the OE filer, then you would likely see a gain... or rather less loss... by replacing the OE system with a free flowing one. Fuel economy is minimally effected, if at all, by less intake restriction.
Convenience/Longevity: No arguing with the "forever" aspects of a OCG. When it gets dirty, you clean it. No more buying filters. That's a good thing. BUT, OCG isn't the only game in town in that regard. There are cleanable dry media filters too, most of which filter better than OCG (though don't flow as well)
Filtering Ability: This is the sticky wicket IMO. Do OCG filter well? IMO, and compared to other media, no. Do they filter "well enough" for many applications? Generally yes, IMO. A street car not driven in dusty conditions will likely not have short or long term issues related to filtering with OCG. If in a dirty environment, like a working truck, an off-roader or even a car that lived in a dirty place, UOAs seem to show a higher level of silica in the oil. Not good and possibly a long-term killer. In my own case, I had visible grit in the intake tract of my off-road rig that was equipped with an OCG filter. That cured me of OCG. There are plenty of efficiency numbers to compare if you look around. You can buy some 99.9 % dry filters out there. They may not flow quite as well as an OCG of equal size, so if you want performance, the answer is simple; increase the area of the filter media to compensate. Me, I'll opt for filtering ability over a fractional power increase every time for my rigs.
