Using 15W40 diesel engine oil in a motorcycle is not recommended. While the viscosity rating (15W40) may match, there are some important differences between diesel engine oil and motorcycle engine oil that make them unsuitable for each other.
Motorcycle engines operate at higher RPMs and higher temperatures compared to diesel engines. They also have different requirements for lubrication, as motorcycle engines typically have a shared lubrication system for both the engine and transmission.
Motorcycle engine oils are specifically formulated to provide the necessary lubrication and protection for motorcycle engines, clutches, and transmissions. They have additives that prevent clutch slippage and provide better lubrication to handle the higher stress and shearing forces experienced in motorcycle transmissions.
On the other hand, diesel engine oils are designed for the unique demands of diesel engines, which have different combustion characteristics and produce higher soot levels. Using diesel engine oil in a motorcycle may lead to clutch slippage, reduced lubrication performance, and accelerated wear in the engine and transmission components.
Even if the viscosity rating matches, it is essential to use the recommended oil specifically designed for motorcycles as specified in the bike manual. If the recommended motorcycle engine oil is not available in your country, it would be best to consult with a local motorcycle mechanic or dealer for suitable alternatives that meet the required specifications for your bike.