If the car is not designed for 15,000 mile change

interval, don't try it. Some engines have very hot spots which are cooled by engine oil. No matter what oil is used, the oil quality will degrade rapidly. Also, MB engines have oil capacity of 8+ liters. The Toyota engines only have 5 liters. Stay with what the manual tells you to do, and be conservative. Some MB dealers recommend change oils every 7,500 miles now.