Fuel has a shelf life
Different fuels last different lengths of time depending on different storage characteristics. But, in the end, no petroleum-based fuel lasts forever in common real-world storage environments.
Biofuels and other alternatives to gasoline/petrol
Dump gasoline/petrol based cars. Those fuels, especially with today's use of ethanol-additives, won't hold up. Stick to diesel engines. Vegetable oils, animal fat, etc., can be burned in these engines. Sure, it's hard on them, fuel economy goes down, etc. but the engines can run, which is far easier to generate locally than any other modern engine fuel.
Bigger problems exist
Even if you solve your gas problem, you have bigger worries. If you have modern engines, then you have to worry about parts that wear out, sensors that fail, electronics, batteries, rubber and plastic parts that degrade over time, belts, etc. For all practical purposes, none of these parts can be fabricated in your post-apocalypse world.
So your engine has a shelf-life. As do your tires, your fuel lines, your...
Yes, you can restore older engines to life with much care and feeding on the mechanics' part. But that still depends on scarce resources: parts, skilled labor, patience, and spare time.
Focus long term
Use the engines you can get for as long as you can, but don't get attached.
Instead, as soon as your world is stable enough to survive day-to-day on, begin working to decouple your survival from modern vehicles. Clear roads and bridges, dig your pits etc. with the construction machinery while it works. But assume that within the next 5 to 20 years, you will hit a wall where any given vehicle will fail beyond repair for some reason or other. Maybe because you can't scavenge a spark plug that fits. Or that rubber fuel line dry rots. Whatever. But you can't go back to the store and buy a new one.
So focus on phasing out the vehicles so they aren't required for survival. Start learning to breed and ride horses, for example.