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Why Does Engine Oil Need to be Changed?

Dirty used motor oil being drained from the engine of a car into a barrel

Most people who know how to drive are aware that it's necessary to change the oil in a car's engine every so often. In fact, there are garages and department auto repair department that do nothing other than change oil. Changing the oil in cars and other vehicles is big business.

Back in olden times, when I was a young man, getting a car's oil changed every three months or three thousand miles was the norm. Nowadays, most car manufacturers allow much longer oil-change intervals, in some cases as long as one year or 12,000 miles.

One may rightly wonder, therefore, why it's necessary to change the oil at all. Why not just add oil as the car needs it?

Even though today's motor oils are much better than they were when I was young, and even though oil-change intervals are much longer than they used to be, there are still several good reasons the oil in a car's engine must be changed in accordance with the manufacturer's specifications.

Let me explain it to you.


Reason 1: To Protect Your Car's Warranty

Oils may be better than they used to be, but oil changes are still vital enough to protecting a car's engine that in some cases, not doing oil changes in accordance with the manufacturer's specifications can void the warranty on your car. Even if you have no interest in motor oil at all, your financial interest in your car should be enough to encourage you to change its oil on a timely schedule.


Reason 2: Because Some Components of Motor Oil Break Down

The "base oil," which means the primary component from which motor oil is made, is extremely stable and lasts a very long time. But certain additives such as detergents, viscosity improvers, anti-wear additives, and corrosion inhibitors do break down over time and lose their effectiveness.

Of particular importance in multi-grade oils, which are what almost all cars use nowadays, are the polymers that allow the effective viscosity to change with the temperature. The polymer molecules actually change shape with temperature, which is what enables an oil to be multi-grade. Once the polymers start to break down, the oil will not be able to increase its viscosity as the engine heats up. It will, in effect, be "stuck" at the lower viscosity, resulting in dramatically decreased lubricating ability.


Reason 3: Because the Oil Gets Dirty and Contaminated

Part of a motor oil's job is to remove contaminants from the oil and hold them in suspension. These contaminants are both physical and chemical, and many of them are much too small to be removed by the engine's oil filter. That's why fresh oil is honey-colored and used oil is black. The oil is black because it removed contaminants, including lots of carbon, from the engine and is holding them in suspension.

Motor oils have a limited ability to hold contaminants in suspension, which is known as the oil's carrying capacity. Once the oil is carrying all the contaminants it is capable of carrying, it is said to be saturated. At that point, it is no longer capable of removing contaminants, which can lead to engine failure from corrosion, physical damage, or clogging of the oil pathways.


Reason 4: To Prevent Sludge from Building Up in your Car's Engine

Sludge is a catch-all phrase for semi-solid or gelatinous lumps of gunk that build up in a car's engine over time. They primarily consist of spent oil whose polymers have congealed and formed masses, but also contain other physical and chemical contaminants.

Fresh motor oil contains active detergents that can prevent or even help remove sludge. But because the detergents break down over time, spent oil contains little or no active detergents, which allows sludge to build up unimpeded.

When a car's engine develops sludge, it prevents the internal parts from being properly lubricated and dramatically increases wear. If the sludge contains abrasive contaminants, it can also directly damage moving components in the engine. Sludge also clogs the oil passageways in the engine, which can lead to catastrophic engine failure.


Reason 5: Because Oil Filters Have Limited Carrying Capacity

An engine's oil filter removes physical contaminants from the oil. But like oil itself, an oil filter has a limit to how much in the way of contaminants it can hold. That limit is known as its carrying capacity.

When an oil filter reaches its carrying capacity, a bypass valve is supposed to open that will allow unfiltered oil to flow out to the engine. Unfiltered oil is better than no oil at all, after all. If the bypass valve fails to open, then no oil at all will flow through the filter, leading to oil starvation and catastrophic engine failure.


The moral of the story is to change your oil and oil filter at intervals that are no longer than what the manufacturer specifies for the vehicle, and to use quality oils and filters that meet the car manufacturer's standards.