Our O-360 sat for 12 years, on the airframe, filled with Aeroshell's pickling oil. When I say "filled", it contained over 5 gallons of oil. Intake and exhaust ports were blocked off with covers made of lexan (and appropriate gaskets installed). I topped up the oil in the crankcase filler and through the upper spark plug holes as needed.
After a dozen years I drained the oil and made the following observations.
1) oil made it all the way to the top of the accessory case - when I removed the mags I borescoped the accessory case and literally saw oil had made it right up to the top of the case. Everything in the accessory case looked shiny and new and dripping with oil.
2) cylinders looked perfect. A word of advice... If inspecting cylinders with a borescope through the bottom spark plug holes after draining the oil, reduce the intensity of your borescope illumination, otherwise you'll flash-blind yourself. Ask me how I know!
3) while Tim Andres raises an important point about not being able to get the air out of the top of the crankcase, what hasn't been mentioned is that air is absolutely "sealed off" from the outside world. Rust comes from the presence of moisture in the air circulating through a crankcase. The moist air circulates as a result of natural convection resulting from heating/cooling cycles of the environment in which the engine is stored. By filling the engine with oil, we are excluding air and sealing off the passages through which air would otherwise be circulated and exchanged. We've essentially stopped the process by which rust-causing moisture is brought into the engine.
Ideally the engine would be placed on an engine stand, mounted on its crankshaft flange, with oil added through the accessory case and allowed to slowly fill the crankcase from the highest point. If this isn't an option, being very patient and filling a horizontally-mounted engine seems to work pretty well. The preservative oil is very thick so it is best to add it when the OAT is warm and the oil is even warmer.
Oh, one additional point. Our local Shell bulk plant indicated to me that Shell has stopped making their 2F preservative oil. Now they make a concentrate, 2XN, which one adds to their normal 100 weight oil. This doesn't seem to jive with the info I find on Shell Global's website. When searching for 2F fluid on Aircraft Spruce's website, one finds only the 2XN concentrate, along with the following note:
"Note: Aeroshell 2XN fluid superceds [sic] the old 2F fluid."
Makes me happy to have several gallons of 2F fluid on hand...