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Petrol retailers like Z Caltex and BP always mention the additive packages with things like detergent and corrosion inhibitors that go into their petrol fuels .
Using Gull 91 as an example does their fuel contain additive packages that contain detergent/cleaning agents .
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Generally speaking, 91 octane doesn't contain those additives (regardless of brand). Only premium petrol - 95 or 98 octane - does, which is one reason why it costs more to buy. The additives are optional and are not required in order to comply with the minimum government fuel specifications.