Using this Racing Boy Oil Filter Cover for Yamaha FZ150i / 135LC ES 5-speed will make it easy for you to monitor the condition of the engine oil since this oil filter cover comes with see through glass. You can check whether the oil has become dark and shows that it is the time to change it. The proper way to do it is by monitoring the odometer, which you need to check it every 2000km for the oil change. I think this actually satisfy your curiosity to see what is the condition of the engine oil inside your bike.
Thanks Ivan for the pictures and info!
2 more pictures after the jump