Most here seem to know nothing about gasoline. Mid grade is simply regular mixed with 91 octane. Depending on time of the year they put in more butane for easier starting. Also they add more oxygen additive, usually methanol for lower smog production. Our fancy blending machine also calculated the Reid Vapor Pressure. It saved the refinery $200,000 a week and for a while I was the only one who could fix it.
It ran on DOS 6.22 in French, good thing I had a background in DOS.

Premium 91+ octane usually has less than 7 component streams, including higher octane gas made from n-butane and Iso-butane over a catalyst, usually hydrofloric acid. This process helped our airplanes out run the German and Jap planes.

Premium 91+ octane usually has higher amounts of cleaner additives. Older cars like my 77 450 SLC do not have anti-knock circuits so I don't use regular unless I can help it. My turbo and supercharged Mercedes cars all were able to run on regular.

Note higher altitude lowers the need for high octane. A hot engine compartment such as my 450 SLC raises the need for higher octane.