K&N Airforce Pre-Cleaners are foam filter wraps designed to extend the service interval of K&N Filterchargers that are being used in very dusty conditions. Made from large, open-cell foam, Airforce Pre-Cleaners will stop 90% of the dirt yet restrict the filter's airflow by only 2%. Red and yellow Pre-Cleaners are made for use with K&N Filter Oil, and they must be cleaned with K&N Cleaner or soap and water. Charcoal gray colored pre-cleaners are made with smaller foam cells that do not use oil. They are designed to be cleaned with K&N Air Filter Cleaner or soap & water.