I read two threads about this on forums, not sure if it was this one. Someone enabled the menu and bought the sensor which plugged into the existing plug - but nothing. They concluded that our sensor is wired to the instrument cluster but cars which have this feature have it wired to the engine ECU and A8 doesn't have that kind of ECU that say a 2.0TDI has. I am happy to be corrected.
e.g.
http://audiforum.us/threads/oil-leve...i.11207/page-2