sludge

The K&N packaging explains, quite in simple english, that the unit passes 50% more dirt than the average paper/fiber filter. This has been well covered in alt.autos.dodge.trucks over the last few years.

The story goes something like this: Paper filters consistantly clean

98% of the average crap out of the average "outdoor" air, K&N's pass 97%. This does equate to a 50% increase in dirt flow.

The bigger issue is the fact that K&N filters don't filter well until they're actually dirty. The more dirt, the better it filters. Freshly cleaned its no better than an oily cotton sock.

"Ruined"... Nope, I doubt anyone's actually had their engine "ingest" a K&N filter or some equivalent action that you could actually call "ruining". Premature failure due to increased wear due to unusually high foreign matter in the intake is more like it.

K&N's do make sense in many situations. The biggest advantage of the K&N is it takes a lot more foreign matter to "clog". If you're operating tractors/trucks/dirtbikes/jeeps/4-wheelers/etc in conditions where paper filters start restricting airflow nearly instantly, the K&N filter is your best answer.

JS

That's true of any type of filter.

Actually, your best answer is a large, high-flowing pre-filter to catch the bulk of the crud before it gets to the finer, standard filter. That's pretty much "filtration 101". It can be difficult to do in the confines of an engine bay, however.