Somewhat longer. The MSDSs consistently state "indefinitely" when sealed containers of motor oil are stored under normal ambient conditions (in other words no hotter than mammalian biology can sustain without injury).
As I understand it, forever. However, there is some debate about certain additives that are used in the oil. The ones under debate aren't used in every oil, so I see this as a non-issue.
Realisticly, how many years supply do you plan to store? I would say right now I've got a 1.25 year supply for my Ranger and a 4 year supply for my Oldsmobile. I have never even considered or thought of the oil going bad while being stored in my non-climate controlled garage.
I would worry more about what the oil would cost next year as opposed to if the oil I bought now is going to still be good next year.
I have used oil that has been in storage for up to six years - no problem. I like to keep mine in rotation. About two years worth sit out in my shed and deal with temps from about 30 degrees to 110 degrees. The remaining oil I keep in the house, (under the bed, in the closet, behind books on the bookshelf. I counted last night - 584 quarts. My wife is a good woman).
I've got some 20 year old Quaker State 10 W 40 in plastic bottles. The bottles have never been opened, and yet, the little window on the side shows 28 ozs. The plastic bottles are still soft and flexable but are slowly turning brown. My guess is that over time sitting in my Dad's shed in heat and cold of two decades, the oil has seeped through the plastic. I'm not going to use the oil but hang onto it as an experiment. I think as long as the bottles/jugs are kept out of the sun, they'll be fine.
I think the average name-brand passenger oil -- when kept in a dry environment like your semi-finished basement with normal living room temps & humidity, could last possibly 10 years.
I don't put my oils & filters in my unheated Michigan garage anymore. But that's because my stash is now up to a 2-3 year supply for five vehicles.
This disease I acquired from BITOG is malignant - but non-fatal/non-painful to anything but my wallet.