A message flashed for about 10 to 15 seconds on the dash of a Mercedes-Benz Sprinter at work saying something along the lines of "Reduce oil level". It was quick so I'm not sure what it said. The vehicle gets serviced regularly by a well known reputable company. Is this common?
If it happened shortly after a service it's probably been over-filled when the oil was changed. Draining a small amount of oil out of the engine should fix it. If that's not the cause it could be a faulty sensor, and it might need replacing. If it's happening regularly, and you know the oil fill was correct it could be diesel fuel getting into the oil through a faulty injector.