Oil does not go bad sitting in an engine. The reason you should change the oil at a time interval is because of contaminants that enter the oil, mainly moisture. If you store your vehicle outside, the engine heats up in the sun and the air inside expands. At night the air cools and condenses forming water droplets to form in the oil and on the inside of the engine. After the vehicle has sat for a while without running, eventually the inside of the engine begins to rust as there is no protective coating of oil to stop this reaction. This oxidized metal finds its way into the engine oil eventually. This process happens if you keep the vehicle inside as well, but the condensate does not build up as quickly.
My suggestion is if you keep it outside or in an area that has a wide temperature swing to keep changing the oil every 6 months. If you have less humidity, and less of a temperature swing, you can probably get away with 1 year intervals. In either case I would start and run the vehicle up to operating temperature at least once a month or more often if possible. This will do two things. It will dispell the moisture that has built up in the engine by heating it to its steam point and it will coat the internals of the engine with a protective coating of oil. Never just start an engine, let it run for minute and shut it down. This just mixes the water with the oil and does not remove it.
Chris B.