Gasoline oxidizes over time, which produces gums. These turn into a kind of varnish that builds up on engine parts during combustion, and will eventually impair efficiency. How quickly that oxidation happens depends on several factors. Some refiners add chemicals that retard oxidation, others may not. Storage at higher temperatures, or in containers that allow more air exposure will favor oxidation. There are additives called fuel stabilizers that you can buy from auto supply stores which are anti-oxidants; these will extend gas life to over a year. You can often see the signs of oxidation. If you compare fresh gas to oxidized gas, the older gas will look darker and cloudier. This is caused by the gums that have formed. You can sometimes see clumps of the gums in very oxidized gas