Oil Fill Plugs comprises of a body adapted for removable attachment to an oil exit port on a machine casing and when the body is attached to said oil input port, connect the inside of said machine casing to an exit port or ports at the exterior of said body, where a filter element formed from an interstitial plastics material is located so as to completely enclose said exit port or ports of the ventilating passage means whereby air passing through the plug to the machine casing must pass through the filter.