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As the consequence of air quality detenoration and the car overpopulation in Bangkok Metropolitan Area, unleaded gasoline was introduced in 1991 In using unleaded gasoline for cars with soft exhaust valve seats, their recession was found to be excessive after only 3000 - 12 000 km of usage on the highway at 120 km/h Such cars represented about 15 percent of the national car population in 1993 Using an anti-valve seat recession additive is one solution used to prevent valve seat recession An engine test simulating field conditions to evaluate additive effectiveness was implemented Potassium-based and sodium-based additives were tested and the test results led to the conclusion that the effective anti-valve seat recession additive treat rate suitable for car population and driving condition in Thailand was 10 mg of potassium or sodium per kg of fuel Unleaded gasoline with anti-valve seat recession additive has replaced leaded gasoline in Thai market as of 1995 Finally, leaded gasoline was completely phased out as of January 1, 1996 by Government regulation