Uni Filter NU-4126ST 2-Stage Air Filter Will NOT Void Vehicle Warranty. Uni Foam Air Filters use polyurethane foam as the filtration media because of it's superior dirt capturing abilities. "Open Cell" Polyurethane Foam is wetted with specially developed filter oil - (NOT INCLUDED). The "sticky" filter oil is suspended in the path of the dirty air making it impossible for dirt to pass through the depth of the filter. Can be easily washed, re-oiled, and re-used.