Here a K&N filter that was used pretty much only in sand.

Here is the inside of the filter, I took a rag and wiped around the first 1" of the 
filter then tapped it a few times with my hand, see the sand that collected where
the clean area was? Look back at the area where I didn't wipe the oil/sand off.

Your probably thinking at this point the filter was not properly oiled, see the red oil in 
the inlet bell of the carburetor below, that is oil from the filter that ran down the filter
and past a 90 degree elbow and settled out at the bottom of the carb.

On the left is the slide from the carb that was run with a K&N filter the slide on the right is 
out of a carb that was run with a foam filter (from Kiowas Pilot) so much sand had passed
that it actually scratched the chrome off the slide, what do you think the slide bore of the 
carb looks like?

Yes it is like this all the way around, take a close look at the carb/filter oil picture with the 
slide still in the carb, it should look like a mirror.

Anybody that thinks a K&N filter is good when used on a off road machine should take 
K&N's advice and cover the filter with a oiled foam pre filter then again they say their 
filters are better than a foam filter yet sell foam pre filters for their filters, confused yet?
FYI I asked the tech help guys at K&N for flow data when a foam pre filter is used the 
answer was "we have no flow data for our filters wile equipped with a foam pre filter"
wonder why? My guess is because the filter will flow less air than a UNI dual stage oiled
foam filter will.