It is not necessary to flush the engine every time you change the oil in a Yamaha FZ or any other motorcycle unless there is a specific issue or concern. Changing the oil regularly and following the manufacturer's recommended maintenance schedule is generally sufficient to keep the engine running smoothly.
During an oil change, it is important to drain the old oil completely and replace it with the recommended oil grade and quantity specified in the owner's manual. This helps ensure proper lubrication and performance of the engine. Additionally, it is recommended to replace the oil filter as well.
If you notice any signs of contamination or unusual debris in the oil, or if you have experienced an engine problem, it may be a good idea to consult a qualified technician or the manufacturer's service center for further guidance. They can assess the situation and determine if additional steps, such as engine flushing, are necessary.