Hi promine,
Just sharing my thoughts about "reusable performance" air filters like the K&N.
In order for that type of filter to "filter" the dirt as well as a disposable pleated paper filter, you need to spray a specific oil to attract the smaller dirt particles that would otherwise pass through. If you practice that, you need to be extremely disciplined with maintaining the cleanliness of the filter.
Many vehicles have MAP sensors in the intake system, located down-wind from the air filter, including the Ford E-series. It is conceivable that there is a risk of the air filter oil getting on that MAP sensor which will create performance issues and often set the "Check Engine" light.
It is for those reasons that I do NOT recommend a K&N reusable type of air filter for regular street use. I do see value in competition cars driven on a track for the day, without applying that oil. Once back on regular streets, I recommend replacing the K&N with a standard disposable paper pleated air filter.
In my vehicles including my 2007 E350-V10 Phoenix Cruiser, I change my air filter every 10,000 miles regardless of it's visual condition. There are circumstances where I would change the filter more frequently on an as-needed basis, for example driving through a severe dust storm.
Many years ago, we drove through Monument Valley in our first motorhome
HERE. The road was dusty red and the winds swirled the red dust so much that we could not see at times. When we left there, I checked the air filter which had completely turned red. I replaced the air filter at the next opportunity.