It differs from oil to oil. The general perception is that synthetic oils tend to have a longer shelf life than their natural counterparts. That said, Sean Jean's Unforgivable, which I'd say is almost all synthetic, has a horrible shelf life (loses notes & diminishing longevity), while many of the older style perfumes stay unchanged for decades.
Environmental factors also play a large part in shelf life; a tester out of its box, in a shop window or under bright lights will not keep as well as a bottle in a box in a cool, dry environment. Sort of like wine.