Most stock air filters uses paper or foam rubber as a filtering medium. In recent years major manufacturers like K&N and Green have come out with a high-performance, permanent type filter that allows for maximum air flow and protection against contaminants. These filters are made from pleated multi-layered oil soaked surgical cotton fabric. It is claimed that the dirt that sticks to the outside does not block airflow significantly for up to 50,000 street miles. The oil barrier in effect acts as a supplemental filter medium without clogging the filter. The high performance filter uses a cotton wire mesh barrier to reduce air turbulence while allowing high volume airflow. The big thing is that these filters can be cleaned and reused, making them essentially permanent. Should you get one for your family car or SUV? The consensus seems to be not unless you are planning to run in a NASCAR race.