[whoooooo123]

Refined oil products (e.g. the stuff you put in your car) go "bad" because they evaporate. In particular, different substances within the fuel evaporate at different rates, so over time the "mix" degrades away from the optimal ratio. If you put old, degraded fuel into your car it can damage the engine.

You can make fuel last longer by storing it in a cool, dark, airtight place, and there are products you can add to make it last longer, but the general rule is that petrol lasts about 3 months and diesel lasts around 6, or at least that's what I've read.

Entropy, sadly, is an immutable and inescapable law of the universe.

[PaulHoule]

There is a myth that "all gasoline is the same" which is not true because gasoline is not specced to a certain composition but rather to how it burns.

For instance,

https://en.wikipedia.org/wiki/Alkene

is added to some gasoline to raise the octane but gas with a high Alkene (or Olefin) content goes bad quickly compared to other gas.