Gasoline is not usable in it's raw form. At least not in a modern engine. It is usually about 40 octain at best. It needs to be pumped up with iso butane and 'octain enhancers' in order to work.

Sadly these vapor off quickly and this adds to air pollution. Hence the formulation for California users that does not contain the formula that he rest of the country uses. Even the rest of the US has two grads of gas: Summer and winter. The winter gas is more enhanced but the cold weather keeps the vapor levels low. Summer gas is made to be less volatile so it will not vapor off as much.

This is expressed as RVP (Reid Vapor Pressure).

But the older gas you had in that can, with the lid on tight, cannot vapor out and thus cannot oxidize into gunk.