How is premium unleaded gasoline obtained?

The basic ingredients of gasoline are obtained by separation of crude oil or cracking (turning larger carbon chains into smaller ones that can be used). The premium refers to how good and 'pure' the fuel is; it has been cleaned of any damaging particles or unusable carbon fractions. The unleaded refers to the fact that lead has not been added. This was a common practise to help increase the efficiency until it was banned because it caused toxic lead pollution.