Buying premium grades of gas is a waste of money if your engine doesn't require 'em. Except for maybe some extra-super additives, the higher grade and higher priced fuel differs in its octane number, and that refers only to its ability to avoid knock (preignition). Remember leaded gas? Tetraethyl lead was added to boost octane rating, reducing knock by suppressing preignition. More and more "lead" was added as autos (motorcycles too?) got higher compression-ratio engines. Then we got to worrying about what all that lead spewed from the highways was doing to air and water and food. Now anti-knock is refined into the gasoline: refineries must produce better fuels. Knock can be hard to detect in modern engines. Try starting up a hill at moderate throttle (engine under load at low RPM). If a strange rattling noise comes from the direction of the powerplant (front, mid, rear, or wherever you keep your engine), better start springing for a better grade of gas. Knock can cause serious wear and damage. This has been your automotive engineering lecture for today . . .
Love Costco! I think they can offer good prices for good stuff because they buy in heavy-duty quantities (sometimes you have to buy in pretty big packages as well), and they buy smart, and they don't try to produce a giant profit on every sale.