I did a boat load a research and 15w40 is a great choice. I called a shop who rebuilds all Benz and the man I spoke to said synthetic oil cannot properly lubricate the 103 &104 design. He said he has seen this problem so many times where people use synthetic and the cams go up in smoke eventually. So it is better to run the diesel grade oil. If you want more protection in colder climates, simply switch to a lighter weight. NO WONDER WHY THESE AWESOME CARS CAME WITH 15W40 FROM THE FACTORY!!!