G'day folks,
The other thing to consider, especially when storing fuel, is that the higher the octane the faster it loses the octane and goes off.
Umm, no. If it's in an unsealed tank, the octane number can actually
increase slightly, as the more volatile components [aka 'light ends'] evaporate! Loss of these light ends will however make starting more difficult, and the engine will run richer. See attached BP info doc.
High octane number fuels contain less 'light ends' to start with, and will change less quickly with evaporation.
'Going off' of fuels is a separate process, and involves chemical reactions between some of the chemical compounds in the fuel with atmospheric oxygen. 'Unsaturated' alkyl hydrocarbons [ olefins] are the components prone to this.
I remember a few years ago at a stihl service school they had a letter from bp themselves stating that average 91 octane fuel begins to degrade substantially after about 10 days depending on conditions.
Yup, it's been my experience that 91 'goes off' much faster than the higher-octane grades, so I don't buy it for small engine use.
98 is really formulated for fuel-injection systems [and is often higher density, and may run rich], so 95 is the best option for carburetted engines IMHO. Any E10 petrol is just bad news for small engines, and is to be avoided.
At least in Oz, we're not yet in the situation that exists in many areas of the USA and UK, where the only quality small engine fuel [that won't result in exy maintenance issues] available is very expensive prepacked stuff like this;
https://aspenfuel.co.uk/products/aspen-fuel/#a4
