An heat exchanger transfers the heat from engine oil passing through a plurality of tube members to engine coolant within a housing for cooling the engine oil. The tube members are arranged within the housing in groups with the tube members in each group being aligned radially to form a fan shaped core. The cores are arranged circularly, so that the effective surface of the tube members through which the heat of the engine oil therewithin transfers to the engine coolant outside of the tube members becomes maximized.