Let me tell you a story. (I'm good at ramble'n on after all)
The story of a 1997 Super Duty w/ 7.3 Powerstroke turbo tow truck.
Affectionately called 'The Dump Truck'
The second owner (me) picked it up in 1999 (late in the year) with like, 97k on the clock.
I soon found that the air filter (stock) needed to be changed between 12 and 15k miles, at 40 bucks a pop!! So this is every 3 months or so, I started looking into K & N filters, all I cared about was that they were reusable, clean it, let it dry, spray the sticky red stuff on, let it dry (it does not really dry up) and send it.
Remember this is a turbo, that thing will pull air through a shty diaper.
500,000+ miles later......same filter element, cleaned and re-oiled about every 15k(+- a few) miles, do the math, about 30 times (give or take).
I've had the intake tubes and housing duct work off and could see the impeller of the turbo and everything was clean, no filter spray residue, anywhere in the intake duct work,
Now, back then K&N was the only reusable game in town, now there are literally a dozen brands that claim to be 'reusable ', I can only testify to the K&N brand and my experience, use at your own risk, results may very.
Thank You.
Class dismissed.