Why is There a Time Limit–in Addition to a Mileage Limit–on Motor Oil?
Even when you're not driving, chemical changes take place within motor oil that lead to degradation and the need for an oil change. Oxidation is a prime culprit. The interaction between oxygen molecules and motor oil molecules naturally leads to chemical breakdown. Just as oxygen causes a cut apple to brown or exposed metal to rust, it breaks down base oils and reduces motor oil’s effectiveness. Although synthetic base-oil molecules are naturally much more resistant to oxidation than conventional base-oil molecules, they eventually succumb to oxidation’s attack. Oxidation can lead to increased oil viscosity, which negatively affects energy efficiency. It also causes the formation of harmful deposits and sludge.
Your vehicle is also subjected to temperature swings, even when it is parked in the garage. Those temperature swings cause condensation to form inside your engine, leading to water contamination. Not driving for extended periods – or taking short trips that don’t allow the engine to fully warm up – allows water to remain in the oil rather than evaporating and exiting through the tailpipe.
So if you ever exceed your motor oil’s recommended time limit, don’t assume you can keep driving just because you have yet to exceed the mileage limit. All motor oils, even top-quality synthetics, have to eventually be changed regardless of how many miles you drive.