Even the newest m104 engines can still leak. My dad drives a 1997 w140 with the m104, and had the head gasket replaced. This happened a few years ago. The car had very low milage at the time - heck, I think it only has 25k miles now. In defense of Mercedes, the leak had to be exceptionally small, but dad has a very good - perhaps overzealous - little dealership which insists on his car being absolutely perfect.