In general, it is recommended to follow the manufacturer's guidelines and use the recommended oil viscosity for your motorcycle. The numbers in oil viscosity, such as 10W-30 or 10W-40, represent the oil's viscosity rating, with the "W" indicating its winter or cold-start viscosity and the second number representing its operating temperature viscosity.
Using a slightly different oil viscosity, such as 10W-40 instead of 10W-30, is usually acceptable as long as it falls within the manufacturer's recommended viscosity range. However, it's always best to consult your motorcycle's owner's manual or contact the manufacturer to ensure the compatibility of different oil viscosities with your specific motorcycle model.
Factors such as climate, riding conditions, and the motorcycle's engine design can influence the recommended oil viscosity. So, it's essential to consider these factors when selecting the appropriate oil for your motorcycle. Using an oil viscosity that is too thin or too thick for your bike's engine can potentially affect lubrication and performance.
In summary, while it may be acceptable to use 10W-40 instead of 10W-30 in your motorcycle, it's advisable to consult the manufacturer's recommendations to ensure the best performance and longevity of your engine.