how long will Boat motor oil last??

With the rising cost of oils ect just curious as to how long will a sealed container or for that matter will a opened gal. jug of Good quality boat motor oil last? Will it go bad after a period of time No doubt it will last longer unopened that a open capped jug but how long? would it be wise to buy a few ahead of time or am I wasting my money and just buy when I need it?

Tom H

Oil does not go bad. Its been in the ground for years.

Mark
I should have stated That with all of the additives ect does this have an effect on the longivity of the oil.
Tom H

I would not worry about it. From all I have read and seen it does not go bad as long as no contaminants are introduced into it.

Thanks Mark