Test have been done on both ways of draining the oil. All oil pans have a lip on the inside at the drain plug. It doesn't allow all the oil to come out. Sucking the oil out from the top has shown to get out more oil. Also as long as you run the engine first to warm it up all the dirt is suspended in the oil and also comes out. Synthetic oil doesn't make sludge. Most MB dealers use the suction method.