This is the famous air filtration test. I love how at the end, K&N gets upset about the results to which the the author says, " They claimed that the deposit on my test filter was oil (it isn't), and then they said oil does not leave the filter and damage mass air sensors. Honestly, you can't have it both ways. Either oil leaves the filter or it doesn't. Oil will damage a mass air sensor if it gets on the sensor wiring. This is highly documented by TSBs easily available online."
Air Filtration Test
Also, as the tests show and as per K&N's own admission, the K&N filters don't filter better than paper filters. Come on, how do you expect cotton gauze to filter better than sheets of paper? Sure you suck in more air, but your engine is inhaling more dirt as well.
While you're at it, read this first hand experience of a former K&N owner as well.
Threw away K&N's due to high silicon in UOA's - Bob Is The Oil Guy
I still have my K&N's but only just in case if I need a filter in emergency or when my dryflow filter is being rejuvenated.