An oil filter removes contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil. A car's oil filter is located on the side of the engine block, which is the lower, larger part of the engine. An oil pump circulates the oil through the engine. Every time the engine oil circulates, it passes through the filter, which protects the engine by catching any large particles that may damage the engine. It also protects the engine by retaining a small amount of oil when the car is turned off.