Changing from premium to regular fuel

Is there an adjustment that can be made to my engine to accomplish the subject?

I have a 2002 Lincoln Navigator with a 5.4L 4V 300hp engine. Recommended fuel is premium, but I have been sussessfully using mid-grade fuel as recommended by the dealer who sole me the velhicle. I occasionally will add a half tank of regulasr fuel to the half tank of mid-grade with no noticeable adverse affect. On the one occasion I filled the tank with regular fuel, the engine pinged, so I never tried it again. The fuel in my area has 10% ethanol. The vehicle currently has 70600 miles on the odometer.

Thanks in advance for any response.