If dust did get past the filter, how would you be able to see it? It would be inside your motor, not 'visible.'

 

It seems to me if the filter were passing lots of filth, there would be some evidence in the intake tract. With the Airaid system, the first 6" or so past the filter are very easy to see with the filter off.

I have never seen any dust (or filter oil) past the filter in 50,000+ miles and two cleanings with the K&N.

IMO, the K&N may actually filter better when dirty, as the spaces between the fibers get smaller. Less performance, but still effective filtering.

My experience is the thing does stop the dirt when oiled properly.

 

After a week of driving on dirt roads in Idaho this summer. The other two trucks in our group had paper filters. They had to replace their filters after the trip. I just brushed off the excess dirt, lightly re-oiled and popped it back in. I did have a pic of the intake side, but it didn't turn out too well. Just have to take my work on it, but it was very clean and I have not had any MAF problems to date.