Replacement air filters from NWP. Heavy duty air filters give you the highest performance for your Stihl chainsaws, due to their large surface area. These filters have a cloth pre filter, which catches the larger material, and a foam filled core, which catches the fine material. Heavy Duty filters work very well (which is a good thing), but tend to ice up in sub freezing conditions. We recommend using a nylon mesh filter air filter when sawing in frozen conditions. These air filters are oil-less. Filters can load up quickly, so be sure to have a couple of spares handy.