Take it from an old oil guy, Marathon refineries are some of the best in the country. All major oil companies do swaps around the country where they do not have local production/refinery pipeline operations and this is standard practice and relates to operational cost. When you pull into a branded station there is no guarantee the oil company made the fuel, but yes their additives are usually in it. The station that sells the fuel has more to do in maintaining quality after production. Filters maintained/correct filters hoses, tanks not leaking drawing water, weather tight. As far as ethanol it is possible the gas you buy does not have it and likely the pump is labeled ethanol as a CYA to insure the legality is covered for any fuel purchase. Ethanol is actually a positive in the winter as it controls small amounts of moisture that can turn to ice in the fuel and plug filters in below freezing temperatures. Ethanol or not is really not much of an issue unless the mix is incorrect greater than 10% ethanol. There are cheap ethanol test kits on the market now if you have a concern. This is a very simple test.