It's generally recommended to follow the manufacturer's guidelines when it comes to selecting the appropriate oil for your motorcycle. In the case of the Yamaha FZ 25, if the user manual specifies 10W40 oil, it's advisable to use that particular viscosity grade.
While the Motul 10W50 1.5 L oil you mentioned may have similar characteristics, it's always best to adhere to the manufacturer's specifications. The recommended oil viscosity is designed to provide optimal lubrication and protection for the engine components under various operating conditions.
Using an oil with a different viscosity grade may alter the lubrication properties and potentially affect engine performance and longevity. It's important to prioritize the well-being of your motorcycle and stick to the guidelines provided by the manufacturer.