Using 15W40 diesel engine oil in a motorcycle is generally not recommended, even if it is suggested in the bike's manual. While there may be similarities between diesel and motorcycle engines, there are also important differences that affect the lubrication requirements.
Motorcycle engines tend to operate at higher RPMs and have different design characteristics compared to diesel engines. Motorcycle engine oils are formulated to meet the specific demands of these engines, including higher shear stability, better clutch compatibility, and protection against heat and high-speed operation.
On the other hand, diesel engine oils are designed to meet the requirements of heavy-duty diesel engines, which have different operating conditions and lubrication needs. Diesel engine oils may not provide adequate protection for a motorcycle engine, and using them could potentially lead to increased wear, clutch slippage, and other performance issues.
If the recommended 15W40 motorcycle engine oil is not available in your country, it's advisable to consult with local motorcycle experts or dealerships to find a suitable alternative. They can provide guidance based on the available options that are formulated specifically for motorcycle engines and meet the required specifications.