K&N got hit with some bad internet stigma of "i know a guy, and his MAFs got messed up", and unfortunately that has plagued them.
They have an entire department set up to try and fail MAFs with oil, and have even gone so far as to have staff on hand to help with dealership issues if there is in fact a claim of the K&N's causing an issue..
.. none to date
K&N Consumer Protection Pledge
There's a ton of info here.. including testing