A hydraulic oil filter is a device used to remove contaminants from the oil in order to help maintain its properties and reduce wear on components. Hydraulic filters are typically mounted externally or internally depending on the application, and their purpose is to protect sensitive control systems, pumps, motors, air compressors, etc., by removing particles either larger than desired (coarse) or too small for human detection (fine). They also can be used to regulate flow rates of liquid passing through them. The size of particle filtered will depend upon the grade/type chosen as well as what system it's being applied in. Proper installation is necessary prevent excessive contamination buildup which affects efficiency levels of transmission systems utilizing hydraulics so routine maintenance should be done with any filter replacement if applicable.