Looking for a way gasoline can last more than 3-6 months?
Going off the simple zombie apocalypse scenario, I want a way to find vehicles and drive them after they have sat for 2,3... x number of years? Another way might be to “preserve” gas if I was to collect it right after the “event”.
The real-world answer is fuel stabilizers.
The more entertaining answer is that your colony has enlisted a pyromaniac who is slowly building a gasoline re-refinery near the edge of town. He collects bad gas from anything with wheels, knows all sorts of chemistry, has several engines that very loudly test the octane rating of his fuel products and call in zombies from several miles away, and tries to ration himself to one explosion per week. Gas that doesn't make the cut gets reprocessed into napalm for ... stopping zombies. And practicing stopping zombies. His scars are helping the local nursing student develop experience in sterile dressings...
Gasoline itself isn't the problem. Originally, gasoline was a naturally occurring or easily fractionated mixture of hexane, heptane, and a little octane (usually with trace impurities of pentane and nonane and various isomers). Gasoline engines made as late as the 1930s ran on this mix straight out of the oil well in some locations (Pennsylvania, for instance), or straight from the distillery with no additives needed.
Then came high compression, requiring higher levels of the heavier fractions -- "octane number", as much as 100% octane -- for resistance to preignition. And then even higher compression, and leaner mixtures, and more spark advance, all in pursuit of higher power and efficiency, requiring octane numbers, in some cases, well above 100 (one grade of aviation fuel is 115 octane).
Obviously, you can't make a fuel with more than 100% content of octane, but over the period from 1930 to 1960 other additives (tetraethyl lead being the most infamous) were discovered that increased the effective octane number -- now defined in terms of "knock resistance" without actually requiring the presence of octane. It's these additives that are mostly responsible for aging effects on stored gasoline -- some are more volatile than hexane and so preferentially evaporate off, some are more prone to low temperature oxidation, which leads to things like "varnish" formation. All of them make the fuel more prone to dissolve stuff from the tanks and fuel lines.
Bottom line is, gasoline will still burn no matter how long it's been stored (in underground tanks, for instance), it just won't work well in modern engines. Solution: don't use modern engines, find old technology to burn your old gasoline.
Any common car engine built before the Second World War, and most modern aircraft engines, will run fine on additive-free gasoline, including old gas that's been stored underground, potentially for years. Further, those older engines almost always have a carburetor, rather than fuel injection, so they can be fairly readily adjusted to change the fuel mixture to run on ethanol, which is by far the easiest engine fuel to produce new.
Option 1. Use kerosene vehicles, it has the longest shelf life of petroleum fuels and like all fuels can be pushed further with stabilizers and sealed containers. But even better old diesel fuel and motor oil can be refined into kerosene.
Option 2. Use diesel, diesel lasts a lot longer than gasoline and is far more plentiful than kerosene (in the US at least), plus if you do find kerosene, you can cut it with diesel and the engine will burn the mix just fine.
Option 3. Use a variable fuel steam engine, bulkier than a normal engine but you can burn almost anything in it, from fuel oil to ethanol. Or choose a solid fuel steam engine and burn coal or wood chips. As a bonus, both can be used to make potable water; always a bonus for the survivor.
If your zombie apocalypse hits today - fuel stabilizers / reprocessing as already answered, however if it arrives in the near future, and the country it arrives in is phasing out fossil fuels, you may not need to worry about this.
There are potentially net-zero-carbon fuels that many latest gen petrol cars can already run on (or be modified easily to run on) including things like methanol and ethanol. These can be generated renewably from as little as water, co2, and energy, and if that energy is renewable you have a green fuel that's backwards compatible with existing car fleets.
If this fuel is available like current gasoline it will last a very long time in sealed containers. A sealed underground tank full of ethanol at a petrol station should last until the tank fractures. The fuel will marginally decay when exposed to air (it will absorb water from 100% until 95% ethanol), but there should be no problem sucking underground fuel tanks dry for years if they're airtight.
CNG and propane
There are vehicles that run on methane and propane and they are pretty common. I know the Schwans trucks run on propane, as do many city buses. It's pretty easy to find too in a apocalyptic world. You just head out into the country, almost everyone in rural areas has a tank up north where its usually cheaper to heat with than electricity.
Ethanol is the main reason gas "goes bad". But the gas is not really bad. Ethanol gradually takes humidity from the air. In normal use, that's not a problem. But left over time, sufficient water is absorbed from the air to collect at the bottom of the gas tank. Gas and water don't mix, not for long.
So the solution I have seen used is to simply let them separate in a clear container (so water content can be seen), and then pour off the pure gasoline into another container. (BTW, the ethanol stays with the water).
