m6pwr and ard,
Thank you guys for the detailed information that you've given us. I will remove my K&N filter today. For years I've been under the impression that the K&N filters were one of the best filters in the market and I was terribly wrong.
2011 535i (07/2011 build) Imperial Blue, Oyster/Black Interior, Anthracite Wood Trim, ZSP, ZPP, ZP2, ZCV, 2TB, 6NF, 6NR.
Retired: 2009 328i,Titanium Silver, Black Interior, Dark Wood Trim, ZPP, ZLP, 609, 494, AAW Tune.