I think Mike might actually be right here. The EPA reglates unleaded gasoline and has to approve what can and can't be added by refiniries and distributors.
From
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"The Clean Air Act provides EPA with the authority to regulate fuels and fuel additives in order to reduce the risk to public health from exposure to their emissions. The regulations at 40 CFR Part 79 (from the U.S. Government Printing Office) require that each manufacturer or importer of gasoline, diesel fuel, or a fuel additive, have its product registered by EPA prior to its introduction into commerce. Registration involves providing a chemical description of the product and certain technical, marketing and health-effects information. This allows EPA to identify the likely combustion and evaporative emissions. In certain cases, health-effects testing is required for a product to maintain its registration or before a new product can be registered. EPA uses this information to identify products whose emissions may pose an unreasonable risk to public health, warranting further investigation and/or regulation."
Of course, after you buy the gasoline, the EPA has little say in what you can add to it.
Ed