Abstract: A heat exchanger for exchanging heat between engine lubricating oil and engine coolant comprises a heat exchange element made up of a plurality of stacked and spaced plate type heat exchange units. The heat exchange units are provided with successive flow openings through which the oil introduced from an inlet opening flows. A bypass valve is provided to control the flow of the oil passing through a bypass opening which is in communication with the flow openings of the heat exchange units. The bypass valve is of the oil temperature responsive type and adapted to open the bypass opening at oil temperatures below a predetermined level, thereby bypassing much of the oil in the heat exchange units when oil temperature is so low that cooling of the oil is unnecessary.