MT-1 fluids are formulated for non-synchronized manual transmissions used in buses and heavy-duty trucks. These products provide protection against the combination of thermal degradation, component wear, and oil-seal deterioration, which is not provided by lubricants in current use meeting only the requirements of API GL-4 or 5. The API MT-1 fluids should not be mixed with engine oils in the same transmission unit. Transmission manufacturers’ specific lubricant quality recommendations should be followed.