Hello M,
Oil is made by refineries. The base oil product does not have problem, but the "additive package" in the finished oil product eventually oxidizes and the additive settles to the bottom of the container.
Old quarts of oil are nice for "collector's" conversation, but I do not recommend using it especially in a modern vehicle.
Adding ATF is typically used by some (reckless) people as a "cleaner" for very old engines because ATF has aggressive detergents in it. Personally, I do not recommend ATF in any mower equipment engine.
Why? During the life of a typical air-cooled engine, it will develop wear deposits that settle in the bottom of the crankcase sump. This stuff (wear deposits) stays dormant, and it is meant to stay there.
Problems with engine wear will quickly escalate if any detergents are added to the engine oil. This wear deposit gets softened up and circulated with the oil and basically grinds the engine moving parts