First, there is what is known as "cellulose media air filters." These are the conventional paper filters that everyone has used for years. The interesting thing about these filters is that the efficiency with which they filter out contaminants increases as a "dust cake" orms on them. However, as this "dust cake" forms, the efficiency with which they allow air to flow through the engine decreases. So with cellulose filters, the key is to leave them in place long enough for contaminant filtration to increase (usually at least 10,000 miles) to a satisfactory level, while air flow is still at an acceptable level.
Image by Ollie Crafoord via FlickrNext are "foam media air filters" and "cotton gauze media air filters." Foam media filters are like a sponge and use tack oil to grab debris. The twisting channels of a foam filter, which are larger than cellulose filters, ensure that debris can be caught at numerous locations, while air flow is superior to cellulose filters because of the larger channels. Cotton gauze filters also offer superior air flow characteristics. These filters are preferable for applications such as racing engines where air flow is more important than contaminant filtration.
Finally, there are filters made from nanofibers. These are sold by AMSOIL and utilize the same technology used by the military in its Abrams tanks. These filter smaller particles than the other types of filters and typically last as long as 100,000 miles. While these filters are obviously preferable to the others, they are surprisingly inexpensive, costing about the same as comparable foam or cotton gauze filters (although certainly more expensive than conventional cellulose media air filters). If you want to avoid the expense, cellulose filters have been shown to be more efficient than foam and cotton gauze filters, which are perhaps better for racing applications. As with motor oil, it is best to choose an air filter which best suits your particular needs.
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