While it is generally recommended to use the recommended engine oil specified by the manufacturer for your specific motorcycle model, it is possible to use Royal Enfield Bullet engine oil in a Yamaha FZS as a temporary solution or in case of emergencies.
Royal Enfield Bullet engine oil is designed to meet the lubrication requirements of Royal Enfield motorcycles, and it may have different properties compared to the engine oil recommended by Yamaha for the FZS. Different motorcycles have different engine designs and lubrication needs, so using the manufacturer-recommended oil is generally the best option for optimal performance and longevity of your motorcycle's engine.
If you find yourself in a situation where you urgently need to top up your engine oil and cannot find the recommended oil for your Yamaha FZS, using the Royal Enfield Bullet engine oil as a temporary solution should not cause immediate harm to your engine. However, it is important to replace it with the recommended oil as soon as possible.
To ensure the best performance and longevity of your Yamaha FZS, it is always recommended to refer to the owner's manual or consult a Yamaha authorized service center for the appropriate engine oil specification for your motorcycle.