In a nutshell, yes, engine oil can and does expire, whether you’re using it or it’s being stored outside of a vehicle. Exactly how long that takes is highly variable, though.
Many factors contribute to engine oil shelf life. Synthetic oils (a mixture of refined oil and several other compounds) have an average shelf life of up to eight years. Conversely, conventional engine oils are “rougher” on a molecular level and tend to break down faster, with an approximate shelf life of up to five years.
Those estimates can also vary based on whether or not the container has been opened. Whether your oil is fully synthetic, a synthetic blend, conventional, or monograde also makes a difference. Opening the bottle but not putting the cap back on tight enough (or at all) will speed up degradation as well.
Your choice of storage environment also matters. Ideally, you should keep your bottles of engine oil in a clean, dry, safe location (away from children and pets, away from food, etc.) where the temperature won’t drop below 40 degrees Fahrenheit or go above 85 degrees. Under these conditions, oil can remain viable up to, if not a little past, its estimated shelf date. Otherwise it will likely lose effectiveness much sooner.
