There was a mag that did a test on all the then currently available major filters. Those being K&N, PiperCross, Green & OEM.

These tests included a particle test, to see how much shit each was letting thru. Although I can't find the article online, I do remember that K&N although it was the best in the the air-flow tests, was the worst, letting almost twice as much shit thru as it's nearest competitor.

More shit = greater engine damage. The pipercross one took the overall, coming 2nd in the flow tests & 2nd in particle testing, the OEM came 1st for particles but last for flow test.

id deffinatly say it depends on what car

i put a k&n on mine and ive never looked back, my mpg went up alot due to the standard airbox bein way too restricting

also it made a noticable performance difference, good for if you need it lol

now on my mums saxo 1.1i it wouldnt be worth a wank

i actually tried this for a laugh - lets say she wasnt impressed