I was told by a Shell lubrication specialist (admittedly many years ago now) that you should change your oil every year even if the car has hardly moved in that time. The reason given was that engine oils contain several additives including detergents and corrosion preventers as well as the multigrade viscosity modifiers. Some of these deteriorate over time rather than mileage, hence the advice to change annually, which is what I always do. You could argue that the filter could be replaced on a mileage basis (best to drain it and refill it, I suppose) but I always change it.
