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. You can order it with a locking tab and a 30 mesh/500 micron strainer basket. All mounting hardware is included. "Hydraulic Fluid Only" is stamped on the cap to prevent damage caused by using the wrong fluid.