Gas Type by Car: Regular, Premium, Diesel or E85 / Cadillac / STSCadillac STS Gas type by year YearGas type2011Regular2010Regular2009Depends on engine: regular or premium2008Depends on engine: regular or premium2007Depends on engine: regular or premium2006Depends on engine: regular or premium2005Depends on engine: regular or premium Source: U.S. EPA / DOE fuel economy data (fueleconomy.gov).