We're not aware of any specific research regarding different brands of fuel and MPG. All petrol and diesel must meet a specific minimum legal standard, and typically the standard E10 unleaded all comes from the same refineries regardless of brand. Different refining processes and additives are used depending on brand, though. The difficulty with calculating MPG using different brands of fuel is the huge number of variables that can affect the result - temperature, the efficiency of the pump itself, how exact your driving is per tank, etc.