This leads to the Thread I started concerning Washing K&N air filters. The controversy ranged from a scientific test showing that K&N Air Filters allow a lot of stuff to pass through them to a Mechanic who did a lot of work for people who owned "Sand Buggies" and said that they worked.
My experience with a new K&N that replaces the stock Filter Element on my Federal 300D is that after being in the Car and not being cleaned or the oil renewed on it for 10 months of city driving was that I found nothing gritty behind the Filter Element inside of the Air Cleaner Housing. Not even anything sticking to the Oil residue from the blow by that is inside part of the Air Filter housing.
http://www.peachparts.com/shopforum/showthread.php?t=212563&highlight=MAF
I do have plans in the future to work up a filter using Polyester Felt Filter Bag Material; because it is washable. I would most likely leave the K&N in place and hook the other filter to the end of the Air Filter.
Also most companies do not say what Micron level their filter will filter out. The few that I have seen that give this infor have them in the 15 Nominal Micron range.