It's all really one and the same. There are slight variations. The primary difference between brands is the amount of detergent the distributors add to it, but the difference is generally minor. The EPA requires all gas to have a minimum amount of detergent to keep a car's engine clean. So in reality most people pay for what's called "Brand Loyalty" because they feel more comfortable dealing with a particular brand.

Several years ago a study was done in Maryland examining the difference between name brand and generic brand gas. After a slew of tests, they came out to be basically the same.

There's what's called Dealer tank wagon prices which is a gasoline station that sells gasoline under the name of a large oil company which usually includes the expense of trademark rights, credit card usage, and mostly the cost advertising. This is the name brand gasoline, the Mobils, Exxons, etc, that have higher gas prices.

Then there is what's called Rack priced Gasoline which only real expense is the transportation cost to move the gasoline from the refinery to the terminal. There ususally are no advertising expenses involved, or costly trademark rights. And because there is no advertising and it is NOT being sold under the name of a large oil company, it can be sold cheaper, hence the generic brand gasoline.

And as far as Costco's cheaper gas prices, the reason that they are able to sell it cheaper is because they rely on the sales INSIDE the store as well as their MEMBERSHIP FEES to make up the difference for them profit wise. But I understand that there is some kind of arguing, within the industry, going on right now that Costco Gas should NOT be exclusive to it's members at a cheaper price! Rumor has it that they may be forced to offer their gasoline to everyone, regardless if they are a Costco member or not. And that's going to translate into Costco rasing the price of their gasoline if this goes through.