Motor oil sitting unopened on a shelf has an expiration date, typically lasting 3-5 years before its additives start breaking down, even though the base oil itself could theoretically last much longer. Once opened, exposure to air and moisture can cut that lifespan in half, as oxygen slowly reacts with the oil's chemistry and humidity allows water contamination. Extreme temperature swings in an unheated garage accelerate this degradation, causing the additives to separate or settle at the bottom of the container like sediment in old wine. Professional mechanics can spot expired oil by its darkened color or unusual smell, but using degraded oil in an engine provides little protection since the anti-wear compounds and detergents have already lost their effectiveness.