When determining when to change your engine oil, it's generally recommended to consider both the riding distance and the time period. While different manufacturers and mechanics may have slightly different guidelines, I can provide you with some general advice.
For motorcycles, the riding distance is often a more critical factor than the time period alone. However, it's still important to change your oil at regular intervals to ensure optimal performance and engine longevity. In your case, averaging 400 km per month, it's advisable to change the engine oil every 4,000 to 5,000 km or approximately once a year, whichever comes first.
Using fully synthetic oil typically allows for longer oil change intervals compared to conventional oils. Synthetic oils offer better lubrication properties and are designed to last longer under various operating conditions. They are also more resistant to breakdown and degradation.
Keep in mind that riding conditions, such as extreme temperatures or frequent stop-and-go traffic, can accelerate oil degradation. If you regularly encounter such conditions, it may be beneficial to change your oil more frequently, such as every 3,000 to 4,000 km.
Always consult your motorcycle's owner manual for the manufacturer's specific recommendations regarding oil change intervals. They may provide more accurate guidelines tailored to your particular model and engine. Additionally, if you're unsure or want a professional opinion, you can consult a trusted mechanic or service center.