What Happens When You Run Old Oil in Your Car

Learn what happens when you run old oil in your car, why old oil breaks down, and why some car makers recommend more miles between oil changes.

Change the oil in your vehicle every 3,000 miles. Generations of drivers took this statement as gospel, and for good reason. Aside from the obvious need to maintain oil at the optimal operating level, what makes frequent oil changes so important? Do today’s vehicles still benefit from that 3,000-mile refresh? What happens when you run old oil in your car?

What Causes Oil Deterioration

At the heart of an internal combustion engine lies a nasty environment. Each time a piston cycles, heat builds from combustion and friction. Impurities from fuel and air enter the cylinder and mix with carbon (a product of incomplete combustion) and water (a product of proper combustion). The rings, designed to prevent the sudden expansion of gas from rushing past the piston, rake across the cylinder wall.

Any of these substances can get into the oil through a process called blow-by, where the piston rings fail to keep the crank case (containing the oil) sealed away from the contents of the cylinder. Blow-by is caused primarily by starting a cold engine or wear on an aging engine.

As a basic property of matter, engine parts expand as they heat up. The piston and rings in a cold engine remain somewhat loose. If this wasn’t the case, your engine would seize as these parts expand and become wedged in the cylinder. As the engine runs, the heat produced expands them to their ideal tolerances. This happens at about 195 degrees F. Before this point, the undersized parts allow some degree of blow-by, which begins to taint the oil.

Overdue Oil Change Damage

Dirty oil creates additional friction that wears away the cylinder wall. Over time, the increase in diameter of the cylinder and the existence of any irregularities in the cylinder wall also promote blow-by. As wearing accumulates, more blow-by occurs, which accelerates the wear and allows more blow-by. A nasty cycle, indeed. Dirty oil also contributes to wear on bearing surfaces, which reduces oil pressure.

Molecular Mischief

On a molecular level, oil is a chain-shaped compound composed of carbon and hydrogen. These hydrocarbon molecules pumped from the ground contain chains of various lengths. The shorter chains are more likely to succumb to the heat in your engine and burn up. This process contaminates the oil as well, making it thicker and less efficient at protecting your engine from wear. An oil extractor pump makes doing your own oil changes a breeze.

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Signs Your Car Needs an Oil Change

Check your oil every few weeks. A low reading on the dipstick should be topped off, but if your translucent, golden-hued lubricant looks sooty and opaque, change it. You may also see a decrease in fuel economy as your old oil thickens, losing viscosity and some of its ability to lubricate.

As the efficiency of your oil fades the engine can generate more heat, so it runs a bit hotter with the potential to overheat. Lastly, your car may start making ticking sounds or other noises from metal hitting metal with little to no oil film between them. Any of these signs signal the need for an oil change. Here are tips on how to choose the right oil and filter for your car.

More Than 3,000 Miles Without an Oil Change?

Some car manufacturers currently recommend longer intervals between oil changes. Several reasons exist to explain this.

It starts with the government ban on lead as an anti-knock compound. Lead was a primary cause of oil contamination. Newer manufacturing technology allows design with more exacting specifications, resulting in cleaner-burning engines and less blow-by.

Finally, synthetic oil optimizes the size of the hydrocarbon chains. This standardization prevents premature oil burn-up, withstands engine pressure better and creates a thinner, more efficient coating on engine surfaces. All these add up to longer-wearing oil.

See how to change car oil on your own in the video below:

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