Gas Type by Car: Regular, Premium, Diesel or E85 / Cadillac / CT6Cadillac CT6 Gas type by year YearGas type2020Depends on engine: regular or premium2019Depends on engine: premium or regular2018Depends on engine: premium or regular2017Depends on engine: premium or regular2016Depends on engine: regular or premium Source: U.S. EPA / DOE fuel economy data (fueleconomy.gov).