I have a '91 240 with only 188,000 miles. It has a slight tapping in the engine, very minor, but a friend says that you should always switch to a higher viscosity when an engine has such hih mileage. He said something about helping with better compression?? I was wondering if any of you with real high mileage have stayed with something like 10W-30 or have you switched to a higer viscosity over the years???