The present invention provides a method for continuous purification of motor lubricant oil comprising circulating lubricant oil between a motor lubricant oil tank and a motor; transporting contaminated lubricant oil from the motor lubricant oil tank in a cleaning loop. The transport in the cleaning loop comprises adding at least one liquid separation aid to contaminated lubricant oil; supplying contaminated lubricant oil to a three-phase centrifugal separator; continuously separating contaminants from the lubricant oil in the separator and continuously discharging a first liquid phase comprising purified lubricant oil from a liquid light phase outlet of the separator, continuously discharging a second liquid phase comprising solid contaminants from a liquid heavy phase outlet of the separator and continuously discharging a sludge phase from a sludge outlet by the aid of a conveyor screw of the three-phase separator. The method further comprises transporting the first liquid phase comprising purified oil back to the lubricant oil tank. The present invention further provides a system for carrying out the method.