Top Tier Detergent Gasoline is a designation given by a consortium of major automakers — BMW, General Motors, Honda, Toyota, and Volkswagen/Audi — to gasoline brands in the U.S. and Canada that meet minimum specifications for engine cleanliness and performance and do not contain metallic additives. Top Tier gasoline contains higher levels of detergent additives in order to prevent the build-up of engine "gunk," known to reduce fuel economy and engine performance, and possibly lead to mechanical breakdown. The Top Tier designation is separate from the issue of octane levels—in order to get the designation, gasoline companies must pass tests proving defined levels of engine-cleaning effectiveness in all grades of gasoline they sell, whether it is economy (low-octane) or premium (high-octane).[1] However, premium gasolines may contain even higher levels of detergent additives. To date, more than 20 gasoline retailers in the US, and 4 in Canada, offer Top Tier gasoline.
As with any designation, other companies may meet the standard but have not been certified. Can't imagine Sunoco is selling low quality gas.