Project Farm on youtube did a very good test on air filters and while most are about the same the K&N are horrible and allow large amounts of dirt and particles into the engine.So here's the thing. I've had k&n on my 07.5 duramax, 95 silverado 1500, and several other toys. I've never had any issues. I now have a wcfab compound kit with their intake dry filter. Anyway I had heard of all these horror stories about filter oil plugging up the MAF sensor. Never happened to me. After a while I kind of was wondering how much someone would have to put to plug the MAF sensor up so I put twice the oil it needed on there and still never gave me a sensor error. The important thing with the oiled filters is that they are properly oiled all over. Lack of oil will get you dirt in engine.
For what it's worth, the WC fab filter has an outerwear prefilter that would perfectly go over a k&N or spectre filter if anyone is looking for a good prefilter. I believe they sell it by itself.