Motor Oil Viscosity Explained

Close up of fresh engine oil being poured from a bottle

An oil's viscosity, sometimes called its weight, is usually the most prominent specification on the oil's label.

When a jug of oil says "10W-40" or "5W-30, for example, those numbers refer to the oil's viscosity. But what do those numbers mean in plain language?

Simply-stated, viscosity refers to an oil's resistance to flowing at a given temperature. Oils with lower viscosities are thinner and will flow more easily. Oils with higher viscosities are thicker and will flow more slowly.

It may help to think of apple juice as a low-viscosity fluid, and molasses as a high viscosity fluid. The higher the viscosity, the thicker the oil, and the more resistant it will be to flow. You can pour a glass of apple juice a lot more quickly than you could pour a glass of molasses.

Traditionally, motor oil viscosity has been measured using a device called a viscometer, which is basically a cup with a calibrated hole or tube on the bottom. The more time it takes for the cup to empty, the higher the oil's viscosity at that temperature.

This measure of an oil's rate of flowing through a calibrated orifice due to gravity alone is known as the oil's kinematic viscosity. In practical terms, it's a measure of the oil's resistance to deformation due to its own internal molecular characteristics, with no force applied other than that of gravity acting on the oil's own weight. Mathematically, it's the ratio of dynamic viscosity to the density of the oil.

Dynamic viscosity (or absolute viscosity) is a measure of a fluid's resistance to deformation or flow when an external force other than gravity is applied. The deformation can be that caused by moving a foreign body through the fluid (for example, by stirring); or it can be deformation caused by forcing the fluid through an orifice, which is more relevant to vehicle engines. Motor oils with higher dynamic viscosity require more force to move them through the engine so they can get to the parts they need to lubricate.

Single-Viscosity (Monograde) Motor Oils

Once upon a time, all motor oils were single-viscosity or "straight weight" oils. They had only one viscosity rating that was based on their kinematic viscosity at 100° C (212° F). If you bought SAE 30 oil, for example, it had a kinematic viscosity of 30 at a temperature of 100° C (212° F).

That was all well and good if you happened to live someplace where it never got cold. For people who lived in cold climates, not so much. The viscosity of a monograde motor oil increases (that is, it gets thicker) as its temperature goes down. On a cold winter day, SAE 30 oil might get so thick that the car's engine wouldn't even turn over. Worse yet, if it did start, it might be deprived of adequate lubrication until the oil warmed up enough to be easily pumped through the engine.

The "solution" to this problem was an imperfect one: people used lower viscosity oils in the winter. The oils intended for winter use had a "W" after the viscosity rating, such as "20W." They also differed from non-winter oils in that their viscosity ratings were determined using dynamic viscosity tests that simulated their ability to be forced through an engine's many oil passages at cold temperatures.

The problem with using a lighter oil in the winter was that once an engine warmed up, the oil would get warmer; and as the oil got warmer, its viscosity would decrease. Hence, an oil light enough to enable the engine to start and to flow easily at cold temperatures might become too light to adequately lubricate the engine once it warmed up to operating temperature.

This dilemma brought about yet another measurement known as viscosity index, which is an arbitrary, calculated, unit-less measurement of an oil's viscosity stability across a typical engine's operational temperature range. The higher the VI, the more stable the oil's viscosity at different temperatures. A VI of 80 was considered good. A VI of 90 to 100 was considered excellent.

A VI score of more than 100 was impossible back then. 100 was where the scale ended. Nowadays, with the advent of synthetic oils, viscosity index improvers, and advances in conventional oil refining technology such as improved de-waxing, there no longer is an upper limit to VI scores. High-quality conventional motor oils often have VI scores of more than 100, and practically all synthetic oils have viscosity indices of at least 125.

Very few (if any) land vehicles use single-weight oils today. They're still used in lawn mowers, snow blowers, and other power equipment that is used within relatively narrow temperature ranges, as well as in industrial machinery; but for vehicles and power equipment that need to be operated in a wide range of temperatures, multi-viscosity oils are the order of the day.

Multi-Viscosity (Multigrade) Oils

Some time in the 1950's, lubrication engineers discovered that by adding specialized polymers to motor oil, they could dramatically improve the VI, thus making the same oil behave like a low-viscosity oil when it was cold, and a high-viscosity oil when it was hot. Quite sensibly, these polymers were called viscosity index improvers, or VII's for short; and the oils were marketed as "all-season oils."

The polymers themselves work by changing their molecular shape as the temperature rises. At cold temperatures, the molecules are folded or coiled over onto themselves. As the temperature increases, they unfold or uncoil into larger molecules, making the oil thicker. By starting with a low-viscosity oil and adding the right amount of the right polymers, an oil can be created whose effective viscosity increases with temperature.

In order to be labeled as multi-viscosity (or "multigrade") all-season oils, oils are tested for dynamic viscosity at low temperatures using the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests to determine the "W" grade, and tested for kinematic viscosity at 212°F (100° C) using a viscometer to determine the higher grade. They are labeled using a two-part hyphenated number (for example, 5W-30), with the first number followed by a "W" to denote "winter."

When multigrade oils were first introduced, most engines had much looser internal tolerances than modern engines, and 10W-40 and 20W-50 were the most popular multigrade oils. Modern engines are built to much more exacting tolerances, and oils with viscosities in the 0W-20 to 5W-30 range are much more popular manufacturer-recommended grades for gasoline engines these days.

The other reason why lower-viscosity oils are recommended by manufacturers is because low-viscosity oils require less energy to push them through an engine. That results in a small, but measurable improvement in fuel economy. Car manufacturers are facing intense pressure from both consumers and governments to make their vehicles more fuel-efficient, and the improvement that low-viscosity oils provide is part of how they do that. Every little bit helps.

Do Higher-Viscosity Oils Protect Your Car's Engine Better?

This is kind of a loaded question that oil geeks love to argue about. Let me try to present a factual, objective answer.

An oil with a higher kinematic viscosity will have an easier time maintaining the film that separates metal parts and prevents them from touching each other, which is how oil lubricates an engine. The reason is that at the molecular level, kinematic viscosity is basically a function of friction between the molecules of the oil. The higher resistance to deformation of a high-viscosity oil means that the fluid layer between the metal parts of an engine will also be more resistant to deformation. So on the strictly theoretical level, the answer is yes.

On the practical level, however, the engineers who designed the engine know all about oil viscosity; and in their great wisdom, they specified an oil viscosity (or more often, a range of oil viscosities) that they believed would adequately protect the engine for the duration of its warranty. So although an oil at the higher end of the recommended viscosity range may do a better job of protecting your engine in the theoretical sense, it may make little or no difference in the real world.

In addition, we have to consider the effect of dynamic viscosity. A higher-viscosity oil is more difficult for the oil pump to move through the engine. Using a higher-viscosity oil than recommended by the manufacturer could therefore result in less protection if the oil flow rate is too low to remove contaminants or if the oil can't get to all the places in the engine that need to be lubricated.

Finally, every engine and every driver are different. The extra protection afforded by an oil at the high end of the manufacturer-specified range probably wouldn't make any difference in a car driven to church every Sunday by that little old lady who seems to provide most of the used cars on dealers' lots. But it may make a difference in a car driven in, shall we say, a more "spirited" manner, or one that is used as a taxicab, Uber vehicle, trailer tow vehicle, or in other demanding ways. That's why manufacturers often recommend oils in the higher end of the viscosity range for vehicles used in such "severe" ways.

In a nutshell, higher-viscosity oils that fall within the manufacturer's recommended range may theoretically provide better protection than lower-viscosity oils due to their innately greater resistance to deformation. But whether or not they will make much difference in practice is much less certain. As with all things oil-related, the most important things are to stay within the vehicle manufacturer's recommended range and to use a high-quality oil.

Choosing the Right Oil Viscosity for Your Vehicle

The most basic requirement to choosing an oil for your vehicle is to stay within the manufacturer's specifications. This includes the recommended viscosity range as well as any other requirements specified by the manufacturer. These may include a particular API grade in North America or ACEA grade in Europe, or it may require that the oil meet a special manufacturer-specific standard.

If the vehicle's manufacturer recommends only one specific type and grade of oil, then that's the one you should use. Using anything else may jeopardize your warranty if damage occurs that can be traced to the oil. Most of the time, however, manufacturers provide a range of allowable oil grades, often accompanied by a chart that keys the recommended oil grades to the temperature range and manner in which the car will be operated. So how do you choose?

The answer is that both the oils at the lower-viscosity and higher-viscosity ends of the manufacturer's recommended range will have their advantages and disadvantages. Choosing the right viscosity for your car comes down to what's most important to you.

A 0W-20 oil, for example, will help maximize fuel economy; but in theory, it also will provide less protection for your engine than a 5W-30 would. Which is more important to you? If you plan to trade the car in after two or three years, then maybe fuel economy is more important. But if you hope to drive the car forever, maybe improved protection is more important to you.

Another consideration is whether your engine burns oil at the lower end of the viscosity range. Low-viscosity oils are more likely to result in oil burning because the lighter, thinner oils are more likely to seep past the piston rings and through the valve guides into the combustion chamber. Burning oil can result in more carbon deposits on the spark plugs, pistons, and rings, and may shorten the life of the catalytic converter. So if your car burns oil when using 0W-20, you may want to try an oil at the higher range of the manufacturer's recommended viscosities.

As your car ages, its oil needs may change. Very often, a car that did just fine on a lighter oil when it was new will start burning oil as the engine wears. In that case, trying an oil at the higher end of the manufacturer-specified viscosity range probably is worth trying. If the oil-burning persists, is excessive, or came on suddenly, however, then you should have a mechanic check the engine. Oil-burning due to normal wear happens gradually. Oil-burning that starts suddenly usually results from a mechanical problem.

On a related note, one of the most common causes of sudden-onset oil burning is also one of the easiest to fix on most cars: a clogged or malfunctioning PCV valve. It's something that definitely should be checked if your car starts burning oil.

Finally, there are some people who simply don't trust the ultra-low-viscosity oils available today. If you're one of those people, then by all means choose an oil at the higher end of the manufacturer's recommended range. Just be aware that you'll be sacrificing some fuel economy in exchange.