04-06-2014, 10:31 AM
The big difference between oil viscosity requirements between cars and our bikes is cooling. In a liquid cooled engine the coolant does most of the work controlling combustion and cylinder head temperatures. In an air cooled engine the oil is responsible for carrying all that heat away from the cylinders. As a result, the oil in an air cooled engine sees a wider range of operating temperatures.
Back in the day when most bikes were air cooled, manuals typical recommended a heavier weight oil for hot temperatures, while liquid cooled engines can pretty much use the same viscosity year round. I suspect that's the purpose of the vague chart in the service manual showing 10w-40 at the higher temperature range. The manual for my CB750 goes as far as to recommend 20w-50 for temps over 100F.
Back in the day when most bikes were air cooled, manuals typical recommended a heavier weight oil for hot temperatures, while liquid cooled engines can pretty much use the same viscosity year round. I suspect that's the purpose of the vague chart in the service manual showing 10w-40 at the higher temperature range. The manual for my CB750 goes as far as to recommend 20w-50 for temps over 100F.