Actually, it has nothing to do with exposure to the air. Once oil has been run in an engine, it contains sulfuric acid, water vapor and other byproducts of combustion. These can slowly attack the bearings and other surfaces. Also, the long chain molecules begin to break down with heat and mileage. So just letting it sit in the engine for a period of time is not good. The fact that engines burn far more cleanly and don't get as much out of tune as in decades past. So the oil doesn't get as dirty, and can go more miles. But it should still be changed every 6 months due to chemical attack.