The difference in fuel consumption will show up mostly under load, i.e. - when towing. That's when load demand drives the advanced ignition timing.
3 different grades of gasoline are 3 different octane ratings, sometimes with different ethanol content. It has nothing to do with the long-since banned leaded gasoline. Harder working engines, such as the newer, small displacement, turbocharged 4 cylinder engines frequently mandate premium octane levels, as they have high combustion pressures that would knock (pre-ignition) with low octane level.