First, ethanol shortens the life of the elastomers it comes in contact with. That includes a number of fuel system parts, plus the crankcase seals in the case of crankcase induction 2 strokes. Ethanol is also hygroscopic: it absorbs water, which results in the fuel turning yellow and smelling badly. Note that fuel discolouration can also be due to gum formation. The fuel doesn't work very well when it is affected either by water or gum. It is better to avoid fuels that contain either ethanol or methanol (methanol is much more aggressive than ethanol) if possible. Ethanol is used very extensively in the US and Brazil, but is barely used elsewhere in the world (those two countries account for about 80% of global use as a fuel additive).
Second, there are various fuel additives other than ethanol, used around the world as fuel diluents which allegedly improve exhaust emissions, or as octane improvers. The most significant diluents apart from alcohols, are various ethers - especially MTBE, TAME, and ETBE. Metallic substances such as MMT and TEL are sometimes used as octane improvers. All of these additives have disadvantages. The alcohols and ethers are unhelpful to machinery. Some of the metallic additives have adverse air quality effects (notably TEL), and all of them tend to reduce the effectiveness of catalytic converters. Fuels made in some countries, especially the US, often include one or more of these components. None of them except ethanol seems to be used extensively in Australia. Fortunately in Australia it is usual to disclose the presence of ethanol in the fuel, via a sticker on the pump at the petrol station.
Third, high octane fuel can be produced by mixing manufactured high octane oil-based components into the straight-run distilled oil components. Generally these high octane components are produced from crude oil, but whereas the classic way to produce refined oil products was to boil the crude and distill off various lighter components (starting with high octane petrol at the lowest temperature, and working upward through kerosene, lubricating oil, etc), more and more refineries add to their supply of the lighter, higher octane components by breaking down heavier oil components through catalytic cracking and other reforming processes. This is the predominant source of high octane components in Australian fuels. Note that some of the cracked components are prone to forming gum during storage periods, especially if stored at higher temperatures. As a result there is likely to be a greater gum problem with fuels that have their octane improved substantially in this way. Chemical additives can be used to alleviate gum formation, at least to some extent.
