Replace your old and dirty air filter with a Drop-In Air Filter for your Gas Vehicle. In most cases an oe-style drop-in air filter will be made with paper filter media, where an aftermarket air filter typically features a synthetic media, or a cotton gauze design that may be dry or oiled. A drop-in air filter is named after the simple installation as it simply drops into the factory airbox in most applications. Shop for drop-in air-filters from manufacturers like S&B Filters, AFE, K&N Engineering, and others available at XDP. An air filter is a crucial component for your engine and its purpose is to keep harmful airborne dust and debris out of your engine.