It is the 21st century, which of course is going to hell in a handbasket on many things automotive, including many monuments to cost cutting such as the development and production of engines that absolutely require synthetic oil or they go into the business of producing oil sludge. But you are indeed very fortunate to have one of those great 20th century engines, one of that century's finest, that is unfinicky on the question, a fact which is reflected in MB's recommendations on the subject. I would go with the 15/40.