The air filter is the unsung hero of every engine on the road. Without it, dust and debris in the air would wear down your piston rings and cylinder walls like a storm erodes a coastline. As the filter does its job it gets loaded up with dirt, and a dirty filter is going to rob your engine of power and put a dent in your fuel mileage. That’s why you’re supposed to replace the filter every 10,000 to 15,000 miles. And when it comes to replacement air filters, they come in three flavors: OEM paper, oiled cotton gauze and oiled foam. Most OEM filters are made of cellulose, or paper, like a coffee filter. Tiny pores in the paper let air through but keep the vast majority of dirt out. Many modern OEM filters even have a viscous coating to help trap fine dust. The problem is, paper filters are restrictive and those tiny pores plug up quickly. So to get more flow and increase the filter’s dirt-holding capacity, the element is pleated, like an accordian. It only takes up a little more space than a flat filter, but has vastly more surface area and thus more pores, more airflow and a longer service life before it gets stopped up. OEM filters are very effective, but their priority is filte...