Draining via the drain plug is not the lowest point of the engine because of the thread of the drain plug. The topsider way is a great way to easily change the oil without going underneath the car. Especially since you have a new c320, I wouldn't worry about sludge and crud at this point of the engine's lifetime. MB has designed these engines to have their oil changed via suction. How much did you get the OW-40 oil for?