Spectro Performance Oils is anwering your question.
“Yes, synthetic oil can be switched to regular oil in an engine. And vice versa: the oil type can be switched as often as desired. Both synthetic and regular oils lubricate engines internally, providing a barrier that prevents metal-on-metal contact inside the engine.
When synthetic oil started gaining popularity, it was thought that it would harm your engine to switch between it and regular oil. It is now believed that not only is switching harmless, but also in some circumstances it may be advantageous. Some synthetic oils are synthetic blends, which include regular oil mixed with synthetic. As these semi-synthetic oils are a blend of the two types, riders should experience no issues from switching between oil types.
It has been stated that when synthetic oils were first introduced, switching from synthetic to regular oil caused leaking, mostly as a result of different seal-swell rates of the oils. However, these issues have since rectified, and there are currently no problems due to switching back and forth. Spectro Performance Oils.”