A bit of science here what goes on here as I have worked as a gas chromatographer supporting the petro industry. First isooctane by definition has an "octane" rating of 100 and n-heptane is defined as zero as used in the refinery knock engines. If we used pure isoctane (or any other saturated/aromatic), the fuel will be stable "forever". Crude oil has a very small percentage of suitable saturated hydrocarbon so since the demand of gasoline is very high we need to create a suitable fuel from the remaining stock.
Since most of the remaining stock is higher molecular weight we need to "chop" the molecules into usable pieces. This is called "cracking" best yields by Catalytic cracking. This is the Cat in the refinery. The cat works very well but produces a very large fraction of unsaturated hydrocarbons. It is these unsaturated hydrocarbons (aka olefins) that are unable and react over time to form gums and varnishes.
Many different reactions are involved but it is well known that some chemicals can be added to slow these reactions down. This are the additives we are discussing
Do they work? Yes but there are a few addition methods to help them even more
In the refinery, the gasoline fraction from the cat is not only full of olefins but it is not high enough in "octane" for modern cars so we need to blend in high octane stock from elsewhere in the refinery. These other stocks can be created from upgrading the gasoline from the cat. Two of such stock is the alkylate or reformate. Both are high quality, high "octane" and highly stable. For example alkylate is very high in isooctane and reformate is very high in toluene. If your fuel was made from a blend of these very high quality products the fuel will very stable.
Conclusion: Since the higher quality product are more expensive and higher "octane". the refineries will use as much cat gasoline as they possibility can and only use the higher quality products in quantities to meet the minimum octane rating. The high octane fuels will contain lower levels of the bad cat gasoline and thus be more stable. This discussion refers to NON ethanol based fuels. Some refineries use zero bad cat gasoline in their 91 and higher grade fuels. After learning this in the early 90's, I switched to 100% usage of 91 or better octane fuel. Zero fuel related issue on my cars, lawnmower, snowblower etc even without adding fuel stabilizer
