15W-50 seems like an extremely heavy oil to be running in a gas motor. That 50 weight may be too thick. If the manufacturer calls for a certain oil, 5W-30, 5w-20 etc, you better stick to what it calls for, otherwise the oil wont go where it should be, and if its too thin, it wont support the load the bearings place on it. You may want to read up on the design on bearings and such. Theres a whole process that involves designing the bearing for a certain load, but then you must calculate the right oil to support that load, as the bearings should make little to no metal to metal contact. The thicker weight oil supports the load better, but are you sure its getting to where it should be? Im sure the thicker oil holds up longer, but the first few thousand miles, that oil is probably not getting to where it should be. IMO theres no need to run any oil thicker than 10w-30 in newer motors if there is no sign of oil being burned. The tolerances are so tight, the thick oil may not be getting to where it should. This is particularly true on import motors, toyota, honda, isuzu, etc. There was a point when I thought the heavier oil was better for my truck, but then I sat in class and learned that heavier oil is not always good when things are still within spec.

Dan