Japeneese motorcycle manufacturers found the limits demanded by the API TC specifications too loose. Oils meeting the API TC standard still produced excessive smoke and could not prevent exhaust blocking. Therefore the Japanese Engine Oil Standards Implementation Panel (JASO) introduced the following specifications:
JASO FA Original spec established regulating lubricity, detergency, initial torque, exhaust smoke and exhaust system blocking.
JASO FB Increased lubricity, detergency, exhaust smoke and exhaust system blocking requirements over FA.
JASO FC Lubricity and initial torque requirements same as FB, however far higher detergency, exhaust smoke and exhaust system blocking requirements over FB.
JASO FD Same as FC with far higher detergency requirement.
Modern passenger car engine oils contain more and more friction modifiers. While this is the good thing for those segments (reduces wear and fuel consumption) it's bad for the motorcycles. At least for those motorcycles which use engine oil to lubricate their transmission and wet clutch. JASO introduced the MA and MB specification to distinguish between friction modified and non friction modified engine oils. Most four-stroke motorcycles with wet clutches need a JASO MA oil.
JASO MA Japanese standard for special oil which can be used in 4-stroke motorcycle engine with one oil system for engine, gearbox and wet clutch system. Fluid is non-friction modified.
JASO MB MB grade oils are classified as the lowest friction oils among motorcycle four-cycle oils. Not to be used where a JASO MA grade oil is required.