Assuming there aren't any external leaks, and the crankcase ventilation system is working properly, the oil rings may be to blame. This isn't uncommon, in fact, I have two re-ring jobs in the shop right now. The good news is that MB may be willing to help out, but the amount of oil usage must be documented by an authorized dealer. The process is very simple. The dealer will do an oil and filter change and probably reset FSS. They'll ask you to drive at least 600 miles, probably more, and then bring the car back to them. If you need to add oil in the meantime, go ahead. You don't even need to remember how much you add. The car will recognize the replenishment and store the information. Just make sure you're using the same oil as the dealer. When you return to the dealer they'll be able to record information about the engines oil consumption using SDS. They can then calculate if the oil consumption is within specification or not. According to MB, approximately one quart per thousand miles is acceptable.