The potential problem with the MAF has to do with people putting too much oil on the paper filter when "recharging" (ie. cleaning) it. I have used K&Ns for years on other vehicles and never had this problem. You just have to be careful not to get overzealous with the pink oil when periodically cleaning the filter.

My ongoing concern with K&N filters is their effectiveness. I think I do get more power with it in my M3, but I have lingering fears that it lets more grit by than the stock airbox/filter. I continue to use a K&N in my M3, but I would never use one on a vehicle that sees a lot of miles in a more dirty environment.

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'01 X5
'98 M3
'73 /5