If you all remember, about a year ago, I fought hard for K&N filters. Got a
lot of static and even a link to page that showed the use of a $250k machine
to disprove my beliefs. Well, I have to say, with humility, that I no longer
support the use of these and here's why. I posted the below on
alt.autos.ford and got some early feedback:

OP - Just an FYI. My dad and I have uncovered a long standing CEL with
"Sally". After doing the normal things, we finally uncovered the
issue- oil/residue from a K&N filter on the MAF sensor. He's been
without the filter for 2 years now and with a CEL light. After
cleaning the sensor with a cotton swab and alcohol, she runs like
there's no tomorrow and the light is gone. I once stood strong on
these filters, but have since changed my thoughts. I'm not downing
them, but after a lot of research, I think I'll sacrifice a bit of
airflow for dry filtration.

So, I got the following reply:

you think that is bad, try running a K&N on a diesel engine.

or better, yet, don't. the K&N lets very small particles pass, causing a
condition on a diesel
engine called "dusting".
basically what happens is the small particles the filter lets pass smoothes
the edges of the turbocharger vanes, wearing them down, which reducing it's
effectiveness. and if left on long enough will blow the engine up.

I stand corrected...and please accept my apologies....

Steve in AZ