Is it better to use heavier oil in older engines? For example I have a 95 BMW 525i with 113,000 miles. I currently run Mobil 1 5w40 truck and suv. I have heard that older high mileage engines just work better with thicker oil. Even more so with BMW engines?? Because of thise issues I have thought about Mobil 1 15w50 for summer driving? What do you think?