Like Landyacht318 I ran a K&N filter, well actually two on a dual carburetor VW powered dune buggy. They were brand new, and freshly oiled. After half a day I pulled the covers and there was obvious sand contamination on the wrong side of the filter.
ironically I still run a K&N on one of my vehicles. Not because I like it but because it's a Frankenstein vehicle and the only filter that I could easily fit. I know it's not the best for it but it's still better than running nothing at all.