GTI,What type of fuel am I okay using?

My car recommends premium gasoline. I am an everyday driver and don't really care about getting the uttermost performance out of the car. I have been using mid-grade fuel and am using Marvel Mystery oil as a fuel additive in the tank every month or so. Am I okay using the mid-grade, or even regular fuel? Thanks

My car has 23000 miles.
My car has a manual transmission.