The Chrome Filler-Strainer Breather Cap Assembly from Buyers Products keeps contaminants out as you fill your oil reservoir, while acting as a fresh air ventilator to allow the tank to "breathe." It prevents a vacuum from building up in the tank. It contains a quarter-turn filler/breather cap that will filter out contaminants as small as 40 microns. All mounting hardware is included. "Hydraulic Fluid Only" is stamped on the cap to prevent damage caused by using the wrong fluid.