In real life, gasoline has a shelf life of only 3-6 months, and diesel 6-12 months (Source). In long-term games, gasoline spoiling and becoming unusable should be a concern.
To counter this, a survivor should outfit their vehicle with LPG (A.K.A. autogas), which has no shelf life. The conversion should require a propane tank, the appropriate tools, and a high enough mechanics and metalwork skill. An amateur mechanic could install it in the trunk, at the cost of trunk capacity, while a master mechanic might manage to take out the gas tank and squeeze the LPG tank in its stead.
As gas ages, it should degrade your engine faster, and lose mileage. Your engine could perhaps make weird sounds, your exhaust become darker, and your character should remark on it directly.
Propane, in addition to being lootable like it is now, should be refillable at pumps at gas stations like gas is, with the caveat that only a few gas stations stock LPG, due to its low popularity in the US. You could have it be available at two or three stations in urban areas, which would give players incentive to occasionally leave their self-sufficient base in the woods and make a dangerous fuel run in a zombie-infested area.
In addition, you could add the option to run cars and generators on biofuels like ethanol, which you can brew yourself.
