Bel-Ray Foam Filter Oil Bel-Ray Foam Filter Oil is a foam air filter oil for all street, off-road, and racing foam air filter applications. Innovative Bel-Ray technology has produced a foam filter oil unlike any other. It is easy to apply and improves airflow. Ultra-viscous formula forms an oil barrier to trap micron-sized particles that can cause engine damage.Bel-Ray's waterproof air filter oil prevents clogging when wet.