What Does Engine Oil Viscocity Mean?

The viscosity, or thickness, of the engine oil that you use will depend on the year, make, model, and engine type of your vehicle, as well as the manufacturer's recommendations and the operating conditions of the vehicle. The viscosity of the oil is indicated by a number on the oil container, such as "5W-30" or "10W-40". The first number, such as "5W" or "10W", refers to the oil's cold viscosity, or how it flows at low temperatures. The second number, such as "30" or "40", refers to the oil's hot viscosity, or how it flows at high temperatures.

What viscosity of engine oil should I use?

The 'Viscosity' of a oil is a way of measuring the the resistance to flow of a engine oil caused by friction within the molecules that make up the oil. Some of the informal terms used to describe the viscosity of a relatively free-flowing liquid such as water include thin, light, and low. Terms such as thick, heavy, or high suggest a liquid with a high resistance to flow, such as honey. However, these terms are general and difficult to measure. More specific classifications give us a better idea of ​​how liquids move.

Temperature affects how a liquid moves. Imagine how the viscosity of honey would increase sharply at temperatures near freezing and decrease near boiling temperatures. To understand these different reactions, types of viscosity are scientifically classified as kinematic viscosity or absolute viscosity.

What Is Kinematic Viscosity?

Kinematic viscosity, also known as fluidity, is a way to measure how easily a liquid flows. It is a measure of how long it takes for a specific amount of liquid to flow out of a container. This flow property is typically expressed in centistokes (cSt) which is a unit of flow volume over time. To determine kinematic viscosity, the American Society for Testing and Materials (ASTM) uses a method known as the ASTM D445 test. This test uses a calibrated tube called a viscometer and a heating bath. The temperature of the bath is set at either 40°C (104°F) for industrial lubricants like hydraulic fluids, compressor oils or gear lubricants, or at 100°C (212°F) for motor oils. The test oil is placed in the viscometer and heated to the specified temperature. Once the temperature is stable, the oil is drawn into a larger area of the viscometer and allowed to drain. The time it takes for the oil to drain is converted to centistokes (cSt) and reported together with the temperature at which it was determined. It's important to note that when comparing kinematic viscosities of different liquids, the tests must be done at the same temperature and at the same time, otherwise the comparison is not valid. And although kinematic viscosity can be also measured in Saybolt Universal Seconds (SUS or SSU) but it becoming less common, but still when given results in both units, it is important to remember that they are different units of measurement and thus cannot be compared at face value.

Absolute/Dynamic Viscosity

Absolute viscosity, also known as dynamic viscosity, is a way to measure the resistance to flow of a liquid. It is the amount of energy required to move an object through a fluid. A liquid that is easy to move, such as water, will have a lower absolute viscosity, while a thicker liquid, such as honey, will have a higher absolute viscosity. This property is generally expressed in units of centipoise (cP), with higher numbers indicating a greater viscosity. It can be a little tricky to comprehend but just like stirring honey with a spoon requires more energy than stirring water, a liquid with higher viscosity requires more energy to move through it.

Viscosity index

Viscosity Index (VI) is a measure of how much a lubricating fluid's viscosity changes with temperature. A fluid with a high VI will change very little in viscosity as the temperature changes, while a fluid with a low VI will change more significantly in viscosity as the temperature changes.

Having a lubricant with a high Viscosity Index is beneficial in that, it maintains a thicker and consistent barrier between different parts of the engine. This can be especially important in extreme temperatures, where lubricants that have a lower Viscosity Index can struggle to provide enough protection. This helps in protecting the critical parts of the engine and maintaining the smooth performance.

Engine Oil Viscosity FAQs

What is viscosity in lubricating fluids and why is it important?

Viscosity is a measure of a lubricating fluid's resistance to flow. It is affected by the temperature and helps to determine how well a lubricant will flow and protect the engine parts. High viscosity oils are thicker and provide more protection, while low viscosity oils flow more easily but may not provide as much protection for the engine components. Choosing the right viscosity oil for your vehicle is important for proper lubrication and engine protection.

What are the different types of viscosity measurements?

There are several different units used to measure viscosity, including centipoise (cP), centistokes (cSt) and Saybolt Universal Seconds (SUS or SSU). The most common unit of measurement for automotive lubricants is the SAE (Society of Automotive Engineers) system, which uses numbers to indicate the viscosity of an oil. It's important to select the right viscosity oil for your vehicle, as determined by the manufacturer's recommendations.

How does temperature affect viscosity of lubricating fluids?

Temperature plays a significant role in the viscosity of lubricating fluids. As temperatures increase, the viscosity of a lubricant will decrease, making it less thick and more able to flow. On the other hand, as temperatures decrease, the viscosity of a lubricant will increase, making it thicker and less able to flow. That's why, manufacturers recommend different types of oil viscosities for different types of weather conditions. And, it's important to keep in mind that viscosity can be affected by the operating temperature range, and this is important to consider when choosing a lubricant for your vehicle.

What does viscocity mean
What does viscocity mean? Credit: Unsplash


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