Top Tier Detergent Gasoline  Top Tier Detergent Gasoline and Top Tier Diesel Fuel are performance specifications and trademarks designed and supported by several major automakers. BMW, General Motors, Fiat Chrysler Automobiles, Ford, Honda, Toyota, Volkswagen, Mercedes-Benz, Navistar, and Audi support the gasoline standard, while General Motors, Volkswagen, Detroit Diesel, and Navistar support the diesel standard (). Each of these automakers recommends the use of Top Tier Detergent Gasoline in their owner's manuals. Top Tier fuels must maintain levels of detergent additives that result in a higher standard of engine cleanliness and performance as compared to the United States Environmental Protection Agency (EPA) requirement. In addition, Top Tier fuels may not contain metallic additives, which can harm the vehicle emission system and create pollutants. As of 2018, Top Tier Detergent Gasoline is available from 61 licensed retail brands and Top Tier Diesel Fuel is available from 5 licensed retail brands. Licensed Top Tier fuel retailers use a higher level of detergent additives compared to other non Top Tier retailers using only the minimum EPA required detergent additives in order to help prevent the buildup of harmful engine deposits which may reduce fuel economy and optimal engine performance. According to an automotive industry spokesman, the regular use of this type of gasoline results in improved engine life. The Top Tier standards must apply to all grades of gasoline or diesel that a company sells, whether it is economy (low-octane) or premium (high-octane). Detergent additives serve to prevent the buildup of engine "gunk," which can cause a host of mechanical problems. Automotive journalist Craig Cole writes, "Gasoline is an impure substance refined from a very impure base stock – crude oil. It’s an explosive hydrocarbon cocktail containing all kinds of different chemicals. In addition to its own molecular variability, refiners and retailers incorporate additional substances into the mix, from ethanol alcohol to octane enhancers." While General Motors' fuels engineer Andrew Buczynsky claims that no one has identified the exact molecule in gasoline that causes engine buildup, he asserts that consistent use of Top Tier Detergent Gasoline will keep engines cleaner. Engine gunk typically builds up in fuel injectors and on intake valves, and if severe can result in reduced fuel efficiency, acceleration, and power. Left unchecked, engine gunk can also contribute to increased emissions, rough idling, and tendency to stall, and can therefore increase required motor repairs. When fuel injectors accumulate deposits, they do not distribute fuel evenly, creating pockets of too much fuel and too little fuel.