The K&N Airforce Pre-Filter is a specially designed foam filter wrap made to extend the service interval of your K&N Filtercharger when used in very dusty conditions. They are made from large, open cell foam. The Airforce Pre-Filter will stop 90% of the dirt, yet only add a 2% restriction to the airflow of the filter. This makes foam a great pre-filter.  The Airforce Pre-Filter is designed to be used with K&N Filter Oil and can be cleaned with K&N Cleaner or soap & water.