99710703800 Genuine Porsche part. Oil Separator. One of the most common failures in the modern water cooled cars is the Air/Oil Separator, also referred to as the AOS. The AOS is basically a small diaphragm distillery which distills the minute amounts of oil mist in the crankcase, liquefies it and sends it back to the crankcase. The “clean” air is now sent into the air intake (after the throttle body) to be burned off by the engine. When the AOS starts to fail it cannot separate all of the oil out of the mist and some liquid oil is sent to the air intake. A few drops of oil is enough to generate the GREAT SMOKE BOMB for which our cars are famous for. A big smoke bomb once in a while is completely normal, due to the architecture of the flat-6 engine, which may let a drop of oil flow into one of the combustion chambers sporadically. For S (3.8ltr) cars.