Gas can be anywhere from 5 to 25 cents cheaper at Costco and Sam's Club than other stations even next door. How?

My question is about gasoline. The conventional wisdom I’ve always heard is that brand-name gas (Chevron, Shell, etc.) is better for your engine than what is sold at discount/mom-and-pop gas stations.

Consumer Reports explains that gasoline “is essentially a shared commodity, often stored in common containers until it is purchased by a major retailer. After that, the retailer treats the gas with a ...