I just recently installed a Gruppe-M intake, and it uses an oiled conical K&N air filter. I had read the reports of MAF sensor fouling, small reductions in HP, etc. While researching the MAF sensor issues, I came across this. Obviously, K&N has a vested interest in disputing the claim that oiled filters foul MAF sensors. However, I think they provided some good information. If nothing else, it is an interesting counterpoint to the stories we hear. Check it out and make your own determination.
Link to 4 part video:
http://www.knfilters.com/MAF/massair.htm
Salty