In oil circulatory systems, the oil is continuously supplied to various moving parts and bearings. In such systems oil acts both as lubricant and coolant by carrying away heat generated in the bearings/moving parts.

The oil after lubrication is returned to reservoir either directly or through filters. These systems are large, employing reservoirs of capacity ranging from few hundred of litres to thousands of liters. The pumps are heavy duty, intended for continuous running, with flow rate ranging from few tons of LPM to few thousands of LPM.