K&N filters may perform well in full size automotive applications, however they do not fare as well in the R/C world. The ideal setup would be a two stage off road filter.

I use an O.S. model that is similar to offerings by Mugen or OFNA. However, the O.S. unit has a neat setup for fastening it in place. It comes with two o-rings that are just the right size to squeeze the filter neck to the engine, and the filter tube to the filter itself. The other brands are fastened with zip ties, so you would have to cut and replace the zip ties each time you wish to clean your filter. Its a slight detail, and the O.S. filter itself doesn't do any better of a job of filtering, but for me, I like it for the ease of removal/installation.