Originally Posted By: DrDave
There seems to be something going on that common sense doesn't account for. I'm trying to envision what specific bearing surface or motor components require such heavy oil. The only possible thing I could Identify on my Sportster motor was wide ring end gaps.
Dave
It has little to do with common sense, it has to do with engineering. Yamaha, Indian, Ducati, Harley, Triumph, etc all have bikes of recent production that are relatively new that in the right climate call for that grade of oil or even thicker. Indian for their air cooled big twins spec a 20w40, but their liquid cooled Scouts call for 10w60. One of the many possible reasons for thicker oil is simply most have a shared sump system with creates sheer and that is taken into account when picking an oil viscosity. For others, it is the fact that the engines run rather hot being of a certain configuration and air cooled. It could also be a concern for fuel dilution. Lots of possibilities.
There are some racing motors out there that use 60 or even 70 weight oils. I know our race car always got 25w50 racing oil. Why? Because of the fuel and injection system we used.