See my Oil analysis results for real world K&N particle filtration on my Kawasaki ZX6R. • 2021 Kawasaki ZX6R Oil...
After 35+ years and over 500k miles of using K&N air filters I can honestly say they are excellent air filters when properly maintained.
K&N air filters require recharging in order to maintain them correctly over their lifetime. If you can follow simple instructions and maintain an oiled, reusable air filter they will work as they should and will save you money over many years of use. If however you can't follow simple instructions and maintain the element as K&N recommends they will not work correctly.
If anyone is truly interested in how K&N filters work look up David Vizard and his K&N presentation. David is a professional engine builder with over 50 years experience and has performed extensive testing on K&N filters. He knows more about air filtration than anyone you will ever hear on the internet. I personally have over 30+ years and 500k miles of K&N history and when properly maintained they will save money over many years of usage.
How to maintain a K&N:
K&N Filter Clean & Oil - How & why to do it properly
• K&N Filter Clean & Oil...
David Vizard presentation (air filtration starts at about 4:30):
Tuning a NA engine by David Vizard
• Video