The choice of engine oil viscosity depends on several factors, including the manufacturer's recommendations, operating conditions, and climate. While it's important to consult the owner's manual or follow the manufacturer's guidelines for your specific motorcycle, I can provide some general information.
In most cases, 10W-40 engine oil is suitable for a 125cc motorcycle engine. This viscosity grade is commonly recommended by motorcycle manufacturers for typical operating conditions. It provides good lubrication and protection for the engine components across a range of temperatures.
On the other hand, 20W-50 engine oil is generally thicker and better suited for high-temperature environments or engines with higher mileage. It may offer better protection for older engines or those that operate in hot climates or under heavy loads. However, using 20W-50 oil in a 125cc motorcycle engine that is designed for 10W-40 may not be necessary or optimal. The thicker oil may not flow as effectively in colder temperatures, potentially affecting engine startup and oil circulation.
It's crucial to follow the manufacturer's recommendations for your motorcycle's engine oil viscosity. Using the recommended grade ensures that the oil provides the appropriate lubrication and protection for the engine under normal operating conditions. If you have specific concerns or requirements, it's best to consult the motorcycle manufacturer or a qualified mechanic for guidance on the most suitable engine oil for your 125cc motorcycle.