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What are the Purposes of Motor Oil?


Motor oil splashing on large gears

Almost all drivers know that they have to check and change the oil in their cars' engines every so often, and most have at least a vague understanding of oil's role in lubricating the engine. That's a good start, but it's only the tip of the iceberg in terms of what the oil flowing through an engine actually does.

The truth is that motor oil has several purposes, all of which are critical to protecting an engine, extending its life, improving its performance, and maximizing its efficiency. Using the wrong oil or an inferior oil can have both immediate and long-term negative effects on your engine.

Different engines have different needs, and the science of tribology has given us many different lubricants optimized to meet those needs. The basic functions of engine oil, however, are the same regardless of the type of engine the oil flows through. Let's take a look at some of the fascinating things that motor oil does.


The most basic purpose of engine oil is to lubricate the engine. There are many aspects to lubricity, each of which has standardized tests to measure an oil's performance in those areas. In its simplest sense, however, lubrication means reducing friction between and wear of the engine's moving parts. Motor oil does this by forming a layer between the parts so ideally, they never touch each other. They touch the oil instead.

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Without the oil layer separating the moving parts of an engine from each other, the friction between them would generate a tremendous amount of heat. That heat would be so intense and localized that within minutes, or even seconds, the parts would either be unable to move because of thermal expansion, or would actually weld themselves to each other. That's what it means when an engine "seizes."

Almost all cases of engines seizing are oil-related, usually as a result of poor maintenance. Not checking the oil often enough is the most common reason. When the engine oil level gets so low that it can no longer be pumped through the engine, the engine will seize within minutes or seconds. Other common mistakes include using the wrong oil, not changing the oil frequently enough, or not using the correct oil filter for the engine (or using a cheap, inferior filter).

In addition to preventing sudden, catastrophic damage, proper lubrication also reduces long-term wear. In addition to the base oil itself, most oils also contain anti-wear additives such as friction modifiers that further reduce wear. These additives break down over time, which is one of the reasons why oil must be changed. The additives break down long before the base oil does.

Prevention of Corrosion and Chemical Damage

Engines are made of a variety of metals. The heads usually are made of aluminum; the blocks can be made of aluminum, cast iron, or steel; and the crankshafts, camshafts, internal gears, and often the cylinder sleeves are usually made of steel. All of these metals are susceptible to corrosion due to moisture, oxidation, or reactions with various chemicals or with each other. The engine oil helps protect the engine's parts both by forming a protective layer over them, and by using additives designed to neutralize or wash away harmful chemicals.

Any quality engine oil will protect an engine that is used regularly from corrosion. Because gravity eventually causes the oil to run down vertical surfaces, however, engines being prepared for storage or that otherwise aren't going to be used for a while (for example, engines in seasonal vehicles or power equipment) can be better protected by using a fogging oil that adheres more tenaciously to metal surfaces. Depending on the kind of engine, fogging oil is usually applied to the air intake and/or directly into the cylinders through the spark plug holes.

Most fogging oils don't require any special procedures when the engine is next started. The oil simply burns off within a few seconds after the engine is started. Check the fogging oil's label just to be sure, though.

Cleaning the Engine

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One of the most important jobs of motor oil to keep the engine clean. Engine oils contain detergents to loosen deposits from the engine's surfaces and hold them in suspension. They also pick up particulate debris such as soot, microscopic bits of metal, and any dirt or debris that made its way into the crankcase. The larger particles are removed from the oil by the oil filter as the oil passes through it. The smaller particles and chemical contaminants are held in suspension in the oil until it the oil changed.

Both oil filters and the oil itself have finite carrying capacities. When an oil filter can no longer hold any additional debris, a bypass valve in the filter will cause the dirty oil to bypass the filtration media and circulate back into the engine. Dirty oil is better than no oil at all. The oil itself also has a limited capacity to carry chemical contaminants that the filter doesn't remove. When the oil becomes saturated with contaminants, it can no longer remove harmful chemicals. This is another reason why both the oil and filter must be changed at intervals well within those recommended by the vehicle's manufacturer.

Cooling the Engine

The coolant in a liquid-cooled engine (or the air in an air-cooled engine) doesn't reach the parts of an engine that actually produce the heat. That part of the job of cooling is the responsibility of the engine oil. The oil both reduces the amount of heat produced by lubricating the metal parts, and transfers what heat is produced to the engine block and head. From there, the cooling system removes the heat and discharges it to the ambient environment.

Many engines also have oil coolers to regulate the engine oil temperature. The engine oil temperature usually is engineered to be higher than the coolant temperature, however. Among other reasons, this helps assure that any water that gets into the oil will boil off. Unless you have a specific reason and have consulted with the car's manufacturer or a qualified engineer or mechanic, you shouldn't install an oil cooler on an engine that wasn't designed to use one.

Conditioning the Seals

Engines have many gaskets, O-rings, and other seals designed to keep the oil inside the engine; and engine oils contain additives to help keep those seals in good shape. Over time, however, the seals will still wear and may start to leak.

High-mileage oils have additives designed to get some extra life out of older seals that have already started to leak, basically by causing them to swell. Whether or not high-mileage oils are a good solution to this problem (as opposed to replacing the seals, which usually is an expensive job) is a matter of personal opinion. For what it's worth, here's mine.

If you can afford it (or if you can do it yourself), then replacing a leaky seal or gasket is always a better solution than using a high-mileage oil. This is especially true if the car is a classic auto with historical value, or if it's simply a car that you'd like to get some more years out of. But if it's a beater car that wouldn't be worth the cost of replacing the seals, then go ahead and try a high-mileage oil or a leak-stop additive. Maybe it will help you get another couple of years out of the car for very little money.

Whatever you do, I urge you not to use a high-mileage oil, high-mileage oil additive, or a leak-stop additive in any engine that's not leaking. Yes, I know that many oil manufacturers say that it's okay. But they're in the business of selling oil. Using an oil designed to swell seals to stop leaks when the seals aren't leaking is just dumb. That's my opinion, anyway: and it's worth every penny you paid for it.

Motor oil being poured from a jug. Laboratory flasks representing synthetic motor oil. A drain pan being used to catch used oil being drained from a car engine. The open top of an engine coolant reservoir. Mechanic using a rooling creeper to work under a car. A hydraulic floor jack being used to raise a car. Three automobile engine oil filters. A mechanic using a wrench to work on a car engine.

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