Using 10W-40 grade engine oil instead of 20W-40 grade oil in your Honda CB Shine 125cc bike is generally acceptable, but there are a few factors to consider:
Temperature Range: The numbers in the oil's viscosity rating indicate its performance in different temperature conditions. The "10W" and "20W" values represent the oil's cold-start viscosity, with lower numbers indicating better flow in colder temperatures. If you frequently ride in very cold conditions, the 10W-40 oil may offer better cold-start performance.
Operating Conditions: The "40" value represents the oil's viscosity at operating temperature. Both 10W-40 and 20W-40 oils have the same viscosity at operating temperature, meaning they offer similar protection once the engine reaches its normal operating temperature. Therefore, using 10W-40 oil instead of 20W-40 oil should not cause any significant issues in terms of lubrication and protection.
Manufacturer Recommendations: While using 10W-40 oil is generally acceptable, it's always best to consult the owner's manual or check with the manufacturer for their recommended oil viscosity for your specific model and engine. They provide the most accurate information based on the engine's design and intended operating conditions.
Riding Conditions: If you frequently ride in hot climates or engage in more demanding riding conditions, such as prolonged high-speed riding or heavy loads, using a slightly thicker oil like 20W-40 may offer additional protection against heat and wear.
Ultimately, as long as the viscosity grade falls within the acceptable range recommended by the manufacturer, using 10W-40 grade oil in place of 20W-40 grade oil in your Honda CB Shine 125cc bike should not pose any significant issues. However, it's important to follow the manufacturer's recommendations and consider the specific operating conditions and climate in which you ride.