I had a K&N on my 91. 5 W-250 for about 10 years. I started noticing quite a bit of dust on the inside of the plenum between the filter and the turbo. You could run your finger around the inside of the plenum and see where it cleaned off the dust. Your finger would have the fine dust on it. Looked like you ran a pencil lead on your finger.
I took good care of the filter cleaning it more often than it needed because, I live in a desert area of the country.
Called K&N and they gave me all my money back and asked if the filter had done any damage to my engine. Sounded like they were willing to compensate me for any damage. I thought that was very fair of them.
That filter did move a lot of air. With the K&N, I could easily get over 30lbs. of boost. Now, with a stock filter, I can only get 25lbs.
I installed an AFE ProGuard-7 on my '03 because of the better filtration.
There's a good article in the latest issue of the TDR about airfilters.
Also, on the Duramax site, there's an interesting test on airfilters, done by SPICER. The GM stock filter did the best.
Hope this helps. Joe F. (Buffalo)