Abstract: Disclosed is a portable apparatus and method for purifying lubricating oil in remotely separated devices, such as gearboxes and motors on oil well pumping equipment. The apparatus is mounted on a vehicle, such as a trailer, and has separate tanks thereon for holding volumes of oil. A flexible conduit on the apparatus is provided for selective connection to a device. Pump means are provided for removing the lubrication oil from the device and pumping it into one of the tanks on the apparatus. A liquid heater is provided in the apparatus for selective connection to the tanks. Appropriate valve and pump means are associated with the heater for use in circulating the oil through the heater to raise the temperature to between 150.degree. and 175.degree. F. A filtering apparatus is provided with appropriate valves and pump means for cleaning the heated oil.