K&N's do filter fine particles better IF they are correctly cleaned and thoroughly oiled AND they are kept from getting very dirty. Two otherwise identical filter setups in the same environment, one foam and one K&N style gauze fiber, the K&N will filter and flow air better at first, but about a quarter of the way through the service limit of the foam element, the filtration will have equalized by the K&N dropping off to the level of the foam. About half to two-thirds through the useful life of the foam filter, the K&N will have become maxed out and will have become restrictive to air flow AND started to not only allow dirt through unfiltered, it will have begun to release dirt previously trapped into the engine under heavy throttle/high RPM loads.
In either filter, it's the oil that does the filtering, especially of the finer particles. Once the oil absorbs its limit of dirt, only the size of the passageways through the element can limit the debris flow across the filter. The thing with foam is that it holds more oil in the first place, has a greater capacity for capturing dirt, is easier to thoroughly oil, and it's not as important that it be perfectly clean before oiling.
Thanks, gray for detailing that out and makong it very clear.