My own view having sold very many for motorbikes is that K & N filters work fine as long as not over oiled. They wouldn't be such a popular option otherwise. They colour the oil red which allows you to put just enough on such that once soaked in to cleaned material a uniform red tint is acheived. Clearly it's possible for over enthusiasm here though.
When last checked after maybe 15,000 miles with K & N slab filter I rinsed sensor with aerosol cleaner catching residue on clean paper towel which remained completely clean. I will check MAF sensor again though since maybe 10,000 further miles and K & N was cleaned not so long ago.
ETA: just checked and sensor clinically clean as before.
![[Linked Image]](https://i.imgur.com/DRwwF3O.jpg)
Apparently should be checked and cleaned every 10,000 miles so it did need doing.
