Here is a good overview of all of the weird and wonderful "extras" that the fuel company may add to refined autogas as it makes it's way through the distribution chain to your friendly neighborhood gas station:

Additives are gasoline-soluble chemicals mixed with gasoline to enhance certain performance characteristics or to provide characteristics not inherent in the gasoline. Typically, they are derived from petroleum-based raw materials and their function and chemistry are highly specialized. They produce the desired effect at the parts-per-million (ppm) concentration range. (One ppm is 0.0001 mass percent or 1mg/kg.)

Oxidation inhibitors, also called antioxidants, are aromatic amines and hindered phenols. They prevent gasoline components from reacting with oxygen in the air to form peroxides or gums. They are needed in virtually all gasolines, but especially those with a high olefins content. Peroxides can degrade antiknock quality, cause fuel pump wear, and attack plastic or elastomeric fuel system parts, soluble gums can lead to engine deposits, and insoluble gums can plug fuel filters. Inhibiting oxidation is particularly important for fuels used in modern fuel-injected vehicles, as their fuel recirculation design may subject the fuel to more temperature and oxygen-exposure stress.

Corrosion inhibitors are carboxylic acids and carboxylates. The facilities — tanks and pipelines — of the gasoline distribution and marketing system are constructed primarily of uncoated steel. Corrosion inhibitors prevent free water in the gasoline from rusting or corroding these facilities. Corrosion inhibitors are less important once the gasoline is in the vehicle. The metal parts in the fuel systems of today’s vehicles are made of corrosion-resistant alloys or of steel coated with corrosion-resistant coatings. More plastic and elastomeric parts are replacing metals in the fuel systems. In addition, service station systems and operations are designed to prevent free water from being delivered to a vehicle's fuel tank.

Metal deactivators are chelating agents — chemical compounds that capture specific metal ions. The more-active metals, like copper and zinc, effectively catalyze the oxidation of gasoline. These metals are not used in most gasoline distribution and vehicle fuel systems. But when they are present, metal deactivators inhibit their catalytic activity.

Demulsifiers are polyglycol derivatives. An emulsion is a stable mixture of two mutually insoluble materials. A gasoline-water emulsion can be formed when gasoline passes through the high-shear field of a centrifugal pump if the gasoline is contaminated with free water. Demulsifiers improve the water-separating characteristics of gasoline by preventing the formation of stable emulsions.

Antiknock compounds are lead alkyls — tetraethyl lead (TEL) and tetramethyl lead (TML), manganese compounds — methylcyclopentadienyl manganese tricarbonyl (MMT), and iron compounds — ferrocene. Antiknock compounds increase the antiknock quality of gasoline. Because the amount of additive needed is small, they are a lower cost method of increasing octane number than changing gasoline chemistry. Gasoline containing tetraethyl lead was first marketed in 1923.The average concentration of lead in gasoline gradually was increased until it reached a maximum of about 2.5 grams per gallon (g/gal.) in the late 1960s. After that, a series of events resulted in the use of less lead: new refining processes that produced higher-octane gasoline components, steady growth in the population of vehicles requiring unleaded gasoline, and EPA regulations requiring the reduction of the lead content of gasoline in phased steps beginning in 1979. The EPA completely banned the addition of lead additives to on-road gasoline in 1996 and the amount of incidental lead may not exceed 0.05 g/gal.

MMT was commercialized in 1959 and was used in gasoline alone or in combination with the lead alkyls. The Clean Air Act Amendments of 1977 banned the use of manganese antiknock additives in unleaded gasoline unless the EPA granted a waiver. MMT continued to be extensively used in unleaded gasoline in Canada at concentrations up to 0.068 g/gal. (18 mg/L). In 1996, after several waiver requests and court actions by the manufacturer, the courts ordered the EPA to grant a waiver for MMT. Its use is limited to a maximum of 0.031 g/gal. (8.2 mg/L). California regulations continue to ban the addition of manganese to gasoline.

MMT's future in the U.S. is clouded: Its use in gasoline is opposed by environmental groups and the automobile manufacturers. Gasoline containing MMT leaves significant red-orange deposits on spark plugs, catalytic converters, oxygen sensors and combustion chamber walls. The additive manufacturer has developed a large body of data to support its claim that MMT in gasoline does not reduce performance or increase emissions. The auto manufacturers have expressed concerns about shortened catalyst, oxygen sensor, and spark plug life and interference with the performance of the new on-board diagnostic system (OBD; see Chapter 5, Gasoline Engines). The auto industry conducted a large vehicle test of MMT that showed increased exhaust hydrocarbon emissions and increases in other emissions in some vehicles at high mileages in vehicles designed to meet low-emission limits. There have been reports of plugging in high-density honeycomb (brick) catalyst systems with MMT. The additive manufacturer takes exception to the autos’ test results and concerns over catalyst plugging. Some new vehicle owner’s manuals recommend against using gasoline containing MMT. Very little MMT is being used in U.S. gasoline at this time (2003). The controversy over MMT continues.

In many parts of the world, lead antiknocks are being eliminated from gasoline used in vehicles that do not have catalytic exhaust emission controls. This gasoline frequently is referred to as lead replacement petrol. Because many of the refineries in the world do not have the processing capability in place to meet octane number needs without an antiknock additive, MMT is being using in lead replacement petrol. The use of MMT helps protect engines susceptible to exhaust valve recession. Alternatives to using MMT for octane enhancement are the use of oxygenates, alkylates, and aromatics until processing can be constructed. The auto industry also opposes the use of MMT in this gasoline because of concern that this lead replacement petrol will find its way into emission-controlled vehicles. Lead replacement petrol is usually dispensed through wide nozzles to prevent misfueling.

Ferrocene (dicyclopentadienyl iron) has been around for nearly 50 years. It has not been widely marketed as an antiknock, although it has seen limited use in Europe. When combusted, it forms ferric oxides (also known as jeweler's rouge), a fine abrasive. Early studies of ferrocene showed excessive piston ring, cylinder bore, and camshaft engine wear at the concentrations investigated. Recent studies by the auto industry at lower iron concentrations have shown premature spark plug failures at the current recommended concentration of 30 ppm (9 ppm Fe). Concern also has been expressed that the ferric oxide will act as a physical barrier on oxygen sensors and exhaust catalyst surfaces and possibly cause catalyst plugging in modern vehicles. In the U.S., ferrocene cannot be used in reformulated gasoline because of a ban on the use of heavy metals. Further, it cannot be used in conventional gasoline without first obtaining a waiver from EPA, which requires extensive vehicle emission testing.

Anti-icing additives are surfactants, alcohols, and glycols. They prevent ice formation in the carburetor and fuel system (see Carburetor Icing). The need for this additive is disappearing as older-model vehicles with carburetors are replaced by vehicles with fuel injection systems.

Dyes are oil-soluble solids and liquids used to visually distinguish batches, grades, or applications of gasoline products. For example, gasoline for general aviation, which is manufactured to different and more exacting requirements, is dyed blue to distinguish it from motor gasoline for safety reasons.

Markers are a means of distinguishing specific batches of gasoline without providing an obvious visual clue. A refiner may add a marker to its gasoline so it can be identified as it moves through the distribution system.

Drag reducers are high-molecular-weight polymers that improve the fluid flow characteristics of low-viscosity petroleum products. As energy costs have increased, pipelines have sought more efficient ways to ship products. Drag reducers lower pumping costs by reducing friction between the flowing gasoline and the walls of the pipe.