What Is Oil Viscosity?
Oil viscosity refers to an oil’s resistance to flowing simple terms, how easily it moves when poured. While it is an important physical property, viscosity mainly influences how effectively an oil flows through a system and protects its components during operation.
From a technical perspective, viscosity describes the relationship between shear stress (the applied force) and shear rate (the speed of flow), helping determine whether an oil is suitable for specific operating conditions.
For example, let's consider walking through a pool of water and then walking through the same pool but filled with molasses. In the case of the pool filled with water, there is a certain amount of resistance, but you are able to walk through the pool with little difficulty. On the contrary, the pool filled with molasses will make it difficult to walk, and the rate of walking will be very slow.
In the given example, the viscosity of the molasses is higher than the viscosity of the water. In the case of lubricating oil, the higher the viscosity, the more resistance it will have, and the lower the viscosity, the less resistance it will have.
The resistance to flow is neither good nor bad, but it depends on the situation.
Another way to look at the concept of viscosity is to use the analogy of a straw. Drinking water from a large-diameter straw is not a problem, as water flows easily. However, drinking the same water from a very thin straw can be more difficult.
Let’s take the same water and attempt to drink it from the thin straw, but this time let’s add honey to the water. The difficulty in drinking the honey/water mixture from the thin straw is even greater.
In a lubrication system, the oil passages, pumps, and clearances can be compared to a straw, which can be large, small, or anywhere in between. If the oil is too thick, it can cause the oil to be difficult to flow. If the oil is too thin, it can flow too easily, not providing a lubricating film.
Years ago, the engines and machines that required lubricating oils were constructed with greater clearance within the engine. Thicker oils, like straight SAE 40 or 50, were used in these applications because of their ability to provide a lubricating film in a loose tolerance application.
Over the years, the technology has improved, and the machines and engines have been made more efficient, compact, and precise. The clearance in the machines has been minimized, and the lubrication systems have been made more critical. The lubricating oils have been called upon to do more than just lubricate; they must also cool and clean the machine.
These machines require oils that are extremely thick, like molasses in a straw. They will not flow fast enough to get to the critical areas in a timely manner. The lighter oils, like the multigrade engine oils used in cars, will flow like water in a straw. They will cool and lubricate the engine in a matter of a few seconds and still provide the protection needed once the engine has reached operating temperature.
Oil viscosity is not a definite factor. Instead, it varies according to the working conditions. The most important factor affecting oil viscosity is temperature. As the temperature drops, the movement of the molecules slows down. The molecules then stick to each other more. This leads to friction between the molecules. The friction makes the oil thicken. On the other hand, as the temperature rises, the molecules move more. The friction between the molecules reduces. The oil then thins. This is the same effect as water turning to ice at low temperatures or vapor at extremely high temperatures. Due to this effect of temperature on viscosity, it is always quoted alongside the temperature at which it was recorded. The temperatures used to record viscosity are always 40°C and 100°C.
Sometimes, oil viscosity may thicken as a result of very high pressures. The high pressures may come from heavily loaded gearboxes or rolling bearings. The increase in viscosity is very important to ensure that the lubricating film does not get damaged.
Flow conditions are also a consideration. Most lubricating oils are Newtonian fluids, which means that their viscosity is independent of shear rate at a given temperature. But in some lubricating oils, especially those containing polymers or thickening agents, viscosity can be reduced by prolonged shear, particularly if viscosity index improvers have degraded with time.
Oil composition is also a very important consideration. Various base oils have different viscosities and vary in their response to temperature changes. Although additives are used to alter and stabilize the viscosity properties, the base oil is always the primary component. This is why the selection of base oil has a profound effect on the viscosity stability of the lubricating oil throughout its service life.
The viscosity of oil can be determined using two methods: kinematic viscosity and dynamic viscosity.
The kinematic viscosity of oil is a measure of the ease of flow of the oil under gravity. This is the most commonly quoted figure on lubricant data sheets and oil analysis reports. The units of kinematic viscosity are centistokes.
The dynamic viscosity of oil is a measure of the force required to move the oil. This is a measure of the "internal friction" of the oil. The units of dynamic viscosity are centipoise or Pascal seconds. Dynamic viscosity is particularly important when assessing cold start up and high load lubrication. Both of these methods of measuring viscosity are valid.
One of the misconceptions regarding viscosity grades is that they indicate a specific value. The truth is that viscosity grades always indicate a range of values.
For engine oils, the SAE scale is used to rate the viscosity of engine oils according to their behavior at low and high temperatures. Viscosity grades like 5W-30 or 10W-40 indicate the behavior of the engine oil during cold start and at high temperatures, not at a point.
For industrial lubricants, the ISO viscosity grades rate the viscosity of industrial lubricants according to their kinematic viscosity at 40°C. An ISO VG 68 lubricant, for instance, has a certain range of viscosity around the number, not exactly the number.
The viscosity index represents how sensitive the oil’s viscosity is to temperature changes. Oils that have high viscosity indices are less affected in terms of changes in oil viscosity when the temperature goes up or down.
Achieving stable viscosity behavior requires the right balance between base oil quality and additive technology. While additives can enhance performance, they cannot compensate for a poor foundation. 👉 This relationship is explored in more detail in the article Additives vs. Base Oil: What Really Matters in Lubricant Performance, which explains why base oil quality always comes first.
Viscosity has a direct impact on wear protection, energy, heat, and oil life. If it is too low, it may cause failure of the lubricating film, resulting in excessive wear. On the other hand, if it is too high, it may restrict oil flow, resulting in excessive heat and friction.
Hence, it is vital to select the appropriate oil viscosity in order to ensure equipment reliability and long life.
To us at Synergysol Trading, viscosity is more than just a piece of information on a data sheet. Viscosity is a performance criterion heavily influenced by the inherent qualities of the base oils used, as well as the formulation strategy employed. We help lubricant manufacturers, blenders, as well as industrial users, to select the right base oils to enable the viscosity to behave as it should.
The starting point to understanding oil viscosity is to select the right foundation.