For longevity and superior performance you need to use synthetic oil. Compressors take in a lot of moisture and synthetic oil will not mix with the moisture. Additionally piston compressors tend to run hot and carbon will build around the valves using petroleum based lubricants. Synthetic oils minimize this problem. IR markets synthetic oil for there piston units and though expensive it will last for a long time.
I read that you are getting good results but IMO I would go from blend to full synthetic. We are in my comfort zone on this as I was with a well recognized compressor company for many years. I was there when we switched from petroleum to synthetic and witnessed the benefits. Even on very high HP units operating 24/7 we extended the oil change period from monthly to annually. Field issues dropped drastically.