Abstract: A heat exchanger for oil from a machine, and particularly from the main spindle system of a machine tool comprises a heat pipe having a heat absorbing portion which is immersed in an oil tank and a heat dissipating portion which is disposed outside the oil tank. An oil guide surrounds the heat absorbing portion and guides hot oil from the machine tool past the heat absorbing portion. A working fluid sealed inside the heat pipe cools the hot oil by evaporation and dissipates the heat to the surrounding air. An oil weir may be disposed at one end of the oil guide so as to maintain an adequate level of oil in the oil guide when the oil level in the oil tank is below the bottom of the heat pipe.