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Though 4-cycle motorcycle engines may be considered more similar to automobile engines than 2-stroke motorcycle engines, they still have very different performance requirements. Historically, 4-stroke motorcycles have had problems with gear pitting wear in the transmissions and clutch slippage. In many cases, this can be directly attributed to the oil used. Most automotive engine oil is developed to minimize friction and maximize fuel economy. Since the oil for many 4-stroke motorcycles is circulated not only through the engine [as with an automobile], but also through the transmission and clutch, different characteristics are required of the oil. First, a certain amount of friction is necessary to prevent clutch slippage. Second, the oil needs to prevent wear and pitting in the gears of the transmission. These and other essential characteristics are addressed in the standards developed by JASO for 4-stroke engines.
As with the 2-stroke classification, the JASO 4-stroke classification is also divided into grades, MA and MB. MB is lower friction oil, while MA is relatively higher friction oil. Other than friction, the JASO 4-stroke classification tests for five other physicochemical properties: sulfated ash, evaporative loss, foaming tendency, shear stability, and high temperature high shear viscosity (HTHS). Sulfated ash can cause pre-ignition if the oil is present in the combustion chamber. It can also contribute to deposits above the piston rings and subsequent valve leakage. Evaporative loss and foaming reduce the amount of lubrication and protection in the transmission, engine, and clutch. With less shear stability, oil loses its capability of retaining original viscosity resulting in increased metal-to-metal contact and wear. High temperature high shear viscosity tests provide viscosity characteristics and data under severe temperature and shear environments.
