thank you very much for your message. Most oil companies have been phasing out engine oil according to VW specification 50501 because of the low demand. This oil was once meant to be used in heavy duty Diesel engines but due to its outstanding shear stability under high temperature we decided to use it for the RS 6 V8 twin turbo engine. The main reason for this move was the position of the turbo chargers which are pretty far located from the engine block and therefore from the oil supplying pump. After driving under extreme conditions, e.g. with high speed or under high load for a longer period of time and very high outside temperature, the oil pressure could drop to low levels when going back to idle. Theoretically, engine oil to VW specification 50500 is not able to compensate the attributes of VW 50501 but the viscosity range of 0W-40 will stabilize oil pressure when the engine and turbo chargers are extremely hot. Hence, potential engine damage is not really an issue if VW 50500 is being used and there is no necessity to do drain this oil immediately.
The official substitute of VW specification 50501 is actually VW 50400, even it is mostly available in 5W-30, for example but without official endorsement MOBIL 1 ESP FORMULA 5W-30. We recommend to use VW 50400 when the next engine oil service is due (every 15000 km).
Kind Regards & Merry Christmas Hanns Breiter quattro GmbH Kundendienst quattro GmbH