Even the Exxon gas at the Exxon station may not have come from an Exxon refinery. For example, ExxonMobil does not own a refinery anywhere in the SF Bay Area (the sold the one the owned to Valero). The do own one in LA, but that is a long way away so it is better for them to make a trade with someone who does own a local refinery. Maybe ChevronTexaco, who have a large refinery in Richmond, but don't have a refinery in Houston (but ExxonMobil does). So ExxonMobil gets Chevron gasoline to sell in their Exxon and Mobil Stations in the Bay Area and ChevronTexaco gets Exxon gas to sell in their Houston stations.