You need to use a DFI oil, but not necessarily pure synthetic. I’m not sure if Merc. makes anything but 100% synthetic anymore? What year Opti. are you running?
Don’t be a cheapskate. Oil is the life blood of your engine. Use the full synthetic Merc oil. If your to concerned about the cost of injection oil apparently you can’t afford the motor.
I have had 2 Opitmax’s now and have never run anything but the Mercury Optimax/DFI oil.
I would kep running this if I were you. What is the price difference $5-10 a gallon and how many gallons do you go through in a summer 2-3??? An extra $30 / year is not worth risking to me.