Oil viscosity is the parameter that plays an important role in lubrication. It changes with temperature, shear rate, pressure, and thickness. Oil viscosity is graded by measuring the time it takes for a standard amount of oil to flow.

Motor oil viscosity is a common term we need to understand completely, and it refers to the ability of an oil to flow. The ability to flow is most often selected by the Society of Automotive Engineers (SAE) numbers.

The numbers are commonly assigned in ranges of 5, 10, 20, 30, 40, and 50. The higher the number, the thicker the oil and vice versa. Thicker oils have more resistance to shearing and losing film strength at higher temperatures.

Oil viscosity differs from one lubricant to another with respect to temperature. One of the factors that affect viscosity is the operating surface. You can easily predict the behavior as well as the design of a mechanical system if you have detailed knowledge of oil viscosity.

What Do Oil Viscosity Numbers Mean

Numbers you find on different oil cans, such as SAE 30 or SAE 10W-30, are viscosity or weight numbers that indicate the thickness of the oil. The higher the number, the thicker is the oil. As earlier mentioned, SAE stands for Society of Automotive Engineers.

SAE established the grading system, which includes a numerical code system. For many years now the viscosity of lubricating oils for motor vehicles has been specified almost universally by the SAE system.

For automotive gear oils, the specification is referred to as SAE J306.

However, this is not the same as the similar SAE system for engine oil viscosity, SAE J300. Although gear oils and engine oils use different SAE numbering systems, the viscosities that the two systems specify overlap.

The initial viscosities were all single, such as SAE 30. Later on, oil additive technology was introduced by the manufacturers that allow engine oil viscosity to thin at a slower rate.

With that said, there are two viscosity types: one for cold temperature performance and the other one for high-temperature performance.

Single Grade or Straight Weight Oil

A single grade oil does not contain viscosity modifier or polymeric viscosity index improver. The SAE classification (SAE J300) has proven 11 viscosity grades. Six of them are recognized as winter grades with a “W” designation.

The eleven viscosity grades are: 60, 50, 40, 30, 20, 25W. 20W, 15W, 10W, 5W, and 0W. These numbers are usually defined as oil weights. Single viscosity oils are normally referred to straight weight oils.

The dynamic viscosity of single viscosity winter oils is determined at a various cold temperature as established by SAE J300. Depending on the coldest temperature the oils are exposed to; they are given the following SAE viscosity grades: 25W, 20W, 15W, 10W, 5W, or 0W.

Based on the winter grades, the lower the number, and the lower is the temperature a given oil can pass. If a specific engine oil passes at the SAE specifications for 5W and 10W and fails to pass for 0W, the oil is then branded as SAE 5W.

On the other hand, the kinetic viscosity of non-winter grade oils is measured at 212 degrees Fahrenheit in millimeter squared per second. Depending on the viscosity range at the specified temperature, the oils are branded as SAE viscosity 60, 50, 40, 30, or 20.

Multi-Viscosity Oil

Although the viscosity of a single grade oil is every so often sufficient to meet engine requirements for a particular climate or season, it may be insufficient in a different environment, hence the need for a grade better suited to the new conditions.

In variable climate zones, drivers who use single viscosity oils generally change to a higher numbered grade for summer driving and a lower numbered grade for winter. The use of multi-viscosity oils can avoid seasonal oil changes.

Since each W grade oil is defined by a maximum viscosity and a maximum border-line pumping temperature, it is possible for an oil to satisfy the needs of more than one W grade. A multi-graded oil is labeled with only the lowest W graded satisfied.

The SAE classification for multi-viscosity oil consists of two viscosity grades, such as viscosity SAE 10W-30. The first part of the grade (10W) is the comparable grade of the single viscosity oil that features the oil’s weight at cold temperature.

The second part of the grade (30) is the grade of the comparable single viscosity oil that defines its viscosity at 212 degrees Fahrenheit. You should note that both parts of the multi-grade SAE classification are not viscosity values but grades.

SAE J300 defines the two numbers that are used for single viscosity oils. Therefore, a multi-grade oil that is branded as SAE 10W-30 has to meet the grade prerequisite for both 10W and 30 as required by SAE J300.

Here is another interpretation of how to read oil viscosity (SAE 10W-30)

SAE = Society of Automotive Engineers

10W = the viscosity of the oil when measured at 0 degrees Fahrenheit (the “W” means winter grade)

30 = the viscosity of the oil when measured at 212 degrees Fahrenheit

In other words, SAE 10W-30 has a base rating of 10 when cold. Therefore, it will flow freely at temperatures as low as -20 degrees Fahrenheit. Usually, oil thins out when heated because it becomes thinner, its viscosity number becomes lower.

An additive package comprising polymers is blended into the oil. Polymers expand when heated, so the oil actually maintains its viscosity to the point where it is equal to what a hot SAE 30 oil would be. This ability to resist change in viscosity as the oil heats up is referred to as viscosity index.

Motor Oil Viscosity Table

Oil Viscosity vs. Temperature Chart

Anton Paar, 2018, Viscosity of Engine Oil, accessed 28 October 2018, https://wiki.anton-paar.com/en/engine-oil/

Oil viscosity change with temperature, as shown in the chart above. In effect, the oil becomes thinner as its temperature rises and this causes problems in engines that operate across a fairly wide range of temperatures. To address this particular problem, motor oils are designed to have a high viscosity index.

Motor Oil Viscosity Index


Usually, choosing the ideal motor oil weight is a compromise. If the weight or viscosity is too low the oil will grow too thin and wear will appear on the metal surfaces of a machine. If it is too high, the machine will struggle to pump the oil, resulting in loss of energy.

An extract oil from naphthenic would show a larger rate of oil viscosity change with temperature compared to an extract oil from a paraffin crude. In this case, the viscosity index is a method of applying a numerical value to this rate of change.

The viscosity index is normally calculated from measurements at 40 degrees Celsius (104 degrees Fahrenheit) and 100 degrees Celsius (212 degrees Fahrenheit). The relationship between the viscosities of fluid at 40 degrees Celsius and 100 degrees Celsius is the viscosity index.

A high viscosity index designates a relatively low rate of change of viscosity with temperature, while a low viscosity index designates a relatively high rate of change of viscosity with temperature.

In other words, the viscosity index is a measure of the extent of viscosity change with temperature. The higher the viscosity index, the less the change, and generally, higher viscosity indices are preferred.

The decrease of the viscosity of a lubricating oil stock with rising temperatures is relatively much more significant for a heavy oil than for light oil. When a viscosity index improver polymer is added to a light oil to increase its viscosity to equal that of heavy oil at a high temperature, the polymer-thickened light oil is considerably more fluid than the un-thickened heavy oil at low temperatures.

This is so provided the viscosity-increasing effect of the polymer is constant or varies weakly with temperature. The variation with temperature of this viscosity-increasing effect is one significant feature of a viscosity index improver.

Here is a table showing the viscosity indices of different types of oils

Low or High Viscosity Index

A lubricant may benefit from having a high viscosity index because of one or more of the below mentioned reasons:

  • Variable ambient temperatures exist
  • Optimum viscosity is unknown
  • To increase energy efficiency
  • Varying speeds and loads exist
  • To increase machine service life
  • To increase oil service life

Lower, cheaper viscosity index lubricants/oils may be appropriate if:

  • The optimum viscosity at the desired operating temperature is not only known but also consistently attained
  • Temperature is constant
  • Loads and speeds are constant

With that said, the viscosity index of motor oil can also tell you valuable info regarding the oil’s formulation, such as the quality and type of base oils. Highly refined lubricants will have similarly higher viscosity indices.

The values of viscosity index are usually found on most data sheets of products for commercially sold lubricants in the market. The viscosity index value may appear as a simple number, but you are strongly urged to consider it when shopping for motor oil.

API Service Ratings


Motor oil or engine oil is also rated on how well it holds up under severe service conditions in the engine. The API service rating is an oil rating system that specifies how well an oil performs under severe engine service.

The American Petroleum Institute (API) service ratings are related to automobile warranty standards. The small engine maker determines which of the ratings are acceptable to meet their engine protection standards.

Apart from the SAE viscosity grades, you should also look for the API service classification. When cars were first engineered, they required API SA, from there they progressed to SB then SC followed by SD and others.

Therefore, if you purchase an engine oil that features API SA rating, then you will be buying an oil that was manufactured for automobiles designed in the 1920s.

Although most vehicles in the US are required to use engine oils that meet a particular API service category, there are some automobile makers who need drivers to use motor oils that meet OEM specific performance specifications.

API Service Category Table

What Weight Motor Oil Should I Use?

Oil Weight

The different kinds of motor oil in today’s market are manufactured for different purposes. To select the ideal motor oil type for your automobile, you first need to understand the importance of additives, classification codes, and viscosity ratings.

  • Additives:

These are elements that are added to the motor oil. They ensure that the engine is free of corrosion, clean, and cool.

  • Classification Codes:

The presence of a starburst symbol on a motor oil simply means that the motor oil satisfies the current fuel economy requirements and engine protection standard of ILSAC.

  • Viscosity Ratings:

Motor oils are rated as well as identified by their viscosities. There are two types of engine oil, which we discussed earlier in this article: straight weight oil and multi-grade oil. Most automobiles are engineered to use multi-grade or multi-viscosity oil.

Every oil can you will come across is marked with a viscosity grade number. If the number is low, the oil is thin, flows easily and can help engines start quickly on cold mornings. If the number is high, the oil is thicker and can help protect the engine as it gets hotter.

If you need to choose the right oil for your vehicle, you should ask yourself these questions:

What kind of oil have you been using?

If your vehicle is running as it should, then there is probably no need to switch to a different brand with a different car oil weight. You will save yourself the trouble of draining all the old oil out of your engine before replacing it with another brand.

What kind of oil does your owner’s manual recommend?

Always confirm if your vehicle is still under warranty. Make sure you use whatever weight of oil the owner’s manual recommends. Using different oil viscosity numbers other than the recommended may invalidate the warranty on a new vehicle.

Some car manufacturers now offer their own brands of motor oil specifically designed for their vehicles. A good example is BMW; this manufacturer reviews their oil formulations now and then. So, it is wise to check the current BMW oil viscosity chart.

Do you live in a very hot or cold climate?

The climate of where you live matters when shopping for motor oil. Between a straight weight and a multi-weight oil, the latter is known to be suitable for a range of temperatures. However, you should always check your car manufacturer recommendation.

Motor oils that work better in cold weather are usually the ones with a lower digit after the “W” or winter grade. Engine oil with a higher digit after the winter grade symbol presents better performance in hot weather.

How worn out is your car engine?

If you have a worn out engine and you have been running 30 weight oil or 40 weight oil, then a multi-viscosity oil will not be reliably thick enough to grease the worn out parts of the engine that are wearing down.

The best course of action is to switch to a heavier straight weight oil in order to keep the oil thick enough to fill the gaps. This is highly recommended when your vehicle gets older and starts to run more roughly or burn up oil more quickly.

Whenever you are out there in search of a motor oil, check for a brand that has the ideal SAE viscosity rating and API certification.

Best Oil Viscosity for High Mileage Cars


If your car is extremely high mileage, it might benefit from an oil that exceeds the manufacturer’s specs for viscosity, such as SAE 5W 30 oil viscosity. High-performance engines will not benefit from high viscosity oils. Heavy oil causes drag and horsepower loss.

The engine of high mileage car is likely to worn out with time. The engine could be suffering from several problems, including gaps due to worn out parts. Seals and gaskets are likely to crack, leading to oil leaks.

You can properly tackle most issues that are associated with high mileage car engines by buying a special type of motor oil that features the ideal oil viscosity. The most recommended oils are usually multi-grade oils.

You can consider the following high mileage oils if your engine goes over the 75,000 miles mark:

1. Mobil 1 High Mileage Oil

This is a specially formulated synthetic oil that features a viscosity SAE 5W-30. It is designed to offer maximum protection for engines that exceed the 75,000 marks. This particular oil helps to protect engine parts from wearing for up to 500,000 miles. It does a great job of protecting rubber seals, ensuring that there are no oil leaks.

2. Mobil 1 Super High Mileage Oil

This is a 10W30 viscosity oil, which is a superior version of the Mobil 1. It offers long engine life under extreme driving conditions. It does a remarkable job of reducing overall wear of high mileage engine, and also reduces oil consumption of the engine.

3. Castrol GTX High Mileage

This is a remarkable high mileage motor oil with SAE 20W-50 viscosity rating. It is a synthetic blend that resists thermal breakdown better than conventional motor oils. This stops damaging deposits from forming under the high heat. This oil also contains seal conditioners, which helps keep engine seals in good condition.

The aforementioned high mileage engine oils are the top three in our suggestion list, but of course, there are others you can consider out there.

Gears must remain separated by a film of oil, even when under heavy loads. As one gear tooth moves against another, oil is forced from between them. This oil must continuously be restocked.

When the lubricant film fails, wear occurs. Gear oils have additives similar to the ones found in engine or motor oils. Additives prevent corrosion and oxidation, reduce friction, limit wear, and prevent foaming.

There are many varieties of gear oils. Some gear oils are thin, such as the ones used in automatic transmissions. Others are thick, such as the ones used in differentials. The oil used in manual transmissions must remain fluid enough to allow for easy shifting in cold weather.

Viscosity for gear oils range from 75 to 250. There are also multi-grades, such 75W-90, and 80W-140, which provide you with both winters as well as hot weather oil weights. You do not have to switch between gear oils when the weather changes during the year.

The “W” in the gear oil viscosity grade is for winter grade, like in engine oil viscosity. Although gear oil viscosity is not the same as that of engine oil, it is rated in centistokes. But the engine and gear oil viscosity overlap. For instance, an SAE 90 gear lube is comparable in viscosity to an SAE 50 or SAE 40 engine oil.

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