I wouldn't trust the GM monitor as much as the MB one.
AFAIK, the MB one uses a quality sensor that checks resistance. The GM one goes by mileage and if you reset it without changing the oil it will let you get away with it.
We can all sit around and argue as to when to change it. How about letting the oil analysis decide? Have it analysed every 50000 and go with that. If the rings are leaking and the engine bearings are shot, 3000 might not be enough. If everything is rignt, maybe 8000 is still too little.