I thought that was one of the main things about k&n filters? That you could re-use them.
You can, which is great. And more airflow means more power! (except it doesn't).
If you think about it, just using common sense, the less restrictive oil filters allow more airflow, and thus more particles, through the filter, while the more restrictive filters let fewer particles, and air, through to the engine. The most common particle allowed to pass though the less restrictive filters is the most common mineral on the planet, silica. It's everywhere. It's in the food you eat, the water you drink, and the air you breath. Sand is made from silica. Put silica into a turbocharger and it becomes literally a sandblaster.
The fine anal folks at the TDI Forum have all the data, and made the initial discovery and link between K&N filters (and similar) and the sandblasting effect. The nearly as anal
Bob's the Oil Guy has a writeup worth reading. And over at the
NICO Club there is a post that K&N would really rather just go away (their responses over the years have ranged from pleading to outright anger).
K&N addressed the issue in the FAQ without actually addressing the issue, in hilarious "PR Speak."
Look closely at the question...
24.
More airflow means you are letting more dirt through, right?
No. Filtration testing measures the percentage of dust retained before the filter reaches a terminal test pressure, often 10" of restriction above initial restriction. We use airflow as a simplified term to explain a more complicated physical process. The more precise description is restriction: K&N air filters create less restriction which helps an engine run better. An engine will only use the air it needs and our air filters do not result in an engine using more air than necessary. Rather, they result in the engine experiencing less restriction. The terms airflow and restriction are inversely related. Our air filters provide either less restriction at a fixed airflow rate; or more airflow as a fixed level of restriction. In neither case is more air being used than necessary.
Notice how they didn't even address the question of letting more dirt through at all, other than a terse "no." Instead they say
'our filters don't flow more air than necessary.' What a hoot. Apparently, we're supposed to believe that air, air, and only air, is what passes through a less-restrictive K&N filter. The problem is, independent tests using strict ISO testing methods show otherwise.
