K&N has been around for a long time.
It was first used only by racers who wanted more air into their engines. The tradeoff was more dirt and grit simply because in simple terms, the holes are bigger - that's how it lets in more air.
Racers didn't care because their engines were constantly being rebuilt- price of admittance.
But, having pretty much saturated the racing market, K&N started hocking it to consumers for it's
longevity, or re-usability, not it's performance.
It was marketed as the 'Million Mile Filter', replacing a few dozen Fram, Autolite or AC/Delco's a consumer would use over the same span of miles. The fact that it offered less protection (which is the opposite of what you want in a street car) was never mentioned, and since cars of the day rarely lasted more than 50k mi., excess wear never really showed up.
Fast forward and today many people use them seeking the performance benefits, disregarding the additional wear. Most of the dirt passed through one is silicate (read sand), and much of this works it's way into your oil, to be circulated around the engine.
There are no free lunches. Many experts have pointed to the K&N as a cause of early MAF failure and there's lots of anecdotal evidence too. If you want the performance of a K&N, be sure to use much less oil than called for when 'refreshing' it, and also consider shortening your oil change intervals as well.
