Just hit 50K on my Focus and the K&N I installed in the first 1K miles was ready to be cleaned per K&N's recommendations (check at 25K and clean/re-oil at 50K). It wasn't too dirty. Cleaning is easy, just spray the "Power Kleen" all over, let it sit 10 minutes, and take a hose and from the inside run water (low pressure) through all portions of the filter until the water ran clear. Let it sit a few hours to dry. Finally, spray (I got the aerosl oil kit b/c I figured it was easier) and coat it until it is all reddish color. Let it dry overnight and re-install. Done!