Abstract: An adapter which enables a person to use different, selected, sizes of inexpensive, readily available, oil or air filters to act as air filters to provide greater air filtration for hydraulic systems. The adapter includes a body having an axial central opening therethrough; an enlarged, cylindrical, central section having first and second opposed surfaces; a first end section extending from the first surface of the central section and having an enlarged, externally, threaded portion; a smaller, externally, threaded portion extending from the enlarged externally threaded portion; a reduced cylindrical section attached to the second surface of the enlarged, cylindrical, section of the body; and a male bayonet locking member fixed to the free end of the reduced cylindrical section of the body for engagement with a female receptacle member attached to a fluid reservoir.