To echo gtmdriver, feel it's desirable to store with clean oil as opposed to laying up with oil used for riding and changing when you get it out again for good weather, as that's often done.
During use the oil accumulates acidic byproducts from combustion, chemically buffered I believe, and less problem when given heat cycles through use. But left static it can cause corrosion mainly from any condensation present that remains on components. Changing the oil, and then not running it, but spinning the motor with plugs out (put some light oil into bores too) to get oil distributed throughout will leave better conditions internally. Then it's ready for restart when you need to.
