It is common practice in many amateur-built engine installations, from Rotaxes to Lycomings, to use K&N filters. They do a wonderful job of filtering the air, and they are reusable. However, they only are able to perform correctly if they are properly cared for. They should be cleaned annually at a minimum with K&N filter cleaner and then lightly oiled with K&N filter oil. Without the oil, which leaves a slight pink color on the filter, they will not filter properly. The one in the photo above was not oiled.