The term 'GAS GRADE' refers to the quality of gasoline. The two crossword solutions for this term are PREMIUM (7 letters) and HIGHEST (8 letters). PREMIUM gasoline is a higher grade of fuel than regular gasoline, and it typically contains detergents that help keep engines clean. It is usually more expensive than regular gasoline. HIGHEST grade gasoline is the HIGHEST quality gasoline available, and it is usually used in high-performance vehicles. It is the most expensive type of gasoline.