Oil Viscosity Index and Viscosity Temperature Relation (2022)

Table of Contents

    Temperature – Viscosity Relation

    Viscosity is a measure of a lubricating oil’s resistant to flow. It is well-known from Hydrodynamic Lubrication Theory that viscosity plays a central role in the lubrication regime encountered in the machine element – the higher is the viscosity, the thicker is the oil film that separates the surfaces from touching each other. However, it is also known that temperature impacts the viscosity.

    Temperature-viscosity characteristics of lubricating oils and/or greases is one of the important determinant for lubricants performances in mechanical systems. Sustainability of lubricating film between contacting bodies in mechanical systems is often critical owing to the highly sensitive nature of lubricants temperature-viscosity relationship. Oil viscosity often declines rapidly with respect to rise in temperature. Loss in lubricants viscosity may lead to severe performance issues of mechanical systems in industry and transportation applications. In tribology systems (e.g. gears, bearings, engines), temperature rise due to friction induced heating is inevitable. Another major source of heat is the high temperature operating environment (e.g. steam/gas turbines). On the other hand, viscosity of lubricant can also be critical for the performance of applications at sub-zero atmospheres. In such cases, lubricants get thicker and add up resistance to the movement of elements. Thereby, neither a thick nor a thin lubricant can be the suitable choice that an engineer can make during the selection of lubricant. An attention to the temperature-viscosity characteristics of lubricants is therefore of practical importance for ensuring the lubricants desirable performance in any particular system.

    Over the years, attempts have been made to develop empirical relationships to describe the lubricants temperature-viscosity behaviors. Reynolds have pioneered in this direction and prescribed an exponential fit to describe the declining trend of viscosity with respect to temperature. The equation appears to be as simple as:

    Oil Viscosity Index and Viscosity Temperature Relation (1)

    (Video) What does Viscosity Index (VI) measure?

    where Oil Viscosity Index and Viscosity Temperature Relation (2) is dynamic viscosity, Oil Viscosity Index and Viscosity Temperature Relation (3) and Oil Viscosity Index and Viscosity Temperature Relation (4) are empirical constants. This equation appears to be simple and can be easily implemented in mathematical models but owing to its highly conservative functionality (i.e. to follow only an exponential curve), it may work well only over a limited temperature range for a limited range of products.

    Another notable equation was developed by Walther in 1928:

    (Video) Why an Oil's Viscosity Index is Important

    Oil Viscosity Index and Viscosity Temperature Relation (9)

    where Oil Viscosity Index and Viscosity Temperature Relation (10) is kinematic viscosity, and Oil Viscosity Index and Viscosity Temperature Relation (11), Oil Viscosity Index and Viscosity Temperature Relation (12), Oil Viscosity Index and Viscosity Temperature Relation (13), and Oil Viscosity Index and Viscosity Temperature Relation (14) are empirical constants. Interestingly, this equation forms the basis of ASTM chart (D341) for viscosity-temperature relationship, and the most general form of this relationship is:

    Oil Viscosity Index and Viscosity Temperature Relation (15)

    Where Z is a transcendental function of kinematic viscosity in cST Oil Viscosity Index and Viscosity Temperature Relation (16), T is temperature in Kelvin scale, A and B are constants. The equation looks unsatisfactory to any math-sophisticated readers; nevertheless, it works well for a large class of petroleum oils over a wide range of temperature Oil Viscosity Index and Viscosity Temperature Relation (17) to Oil Viscosity Index and Viscosity Temperature Relation (18). For further reads, refer to Wright [1969].

    (Video) What is a Viscosity Index ?

    Another model was proposed by Vogel: Oil Viscosity Index and Viscosity Temperature Relation (23) , where x, y, and z are empirical constants. This model proves to be more useful in engineering calculations and predictive numerical models.

    Despite of the development in temperature-viscosity characteristics in empirical terms, some of which are useful for engineering calculations; nevertheless, they often appear to be less useful as a quick guidance for practitioners. The reason behind such ambiguity lies in the need of huge experimental data in order to reliably fit each of these equations. In simple terms, empirical relationships do not satisfy the need of a readymade metric for viscosity-temperature characteristics for a common user of lubricants.

    Temperature – Viscosity Calculator

    Here is a simple calculator that can be used to calculate dynamic viscosity of an oil at operating temperature, if the dynamic viscosities at 2 other temperatures are known (typically the viscosities at 40 °C and 100 °C are taken from the data sheet as reference points, but any two temperatures and viscosities can be used).

    As a rule of thumb, the viscosity of a machine oil fall about 25% with every 10ºC temperature increase. The Reynolds interpolation function is accurate in the vicinity of the reference points. The Vogel equation is most accurate over the whole range.

    Oil Viscosity Index Definition

    Dan and Davis in 1929 introduced a term, viscosity index to compare the oils temperature-viscosity relationships in relative to each other. In early days of industrial lubrication, it was realized that lubricants made from Pennsylvania crude oil is less sensitive to temperature as compared to the lubricants from Golf Coast oil while they both have same kinematic viscosity at 100Oil Viscosity Index and Viscosity Temperature Relation (24). Therefore, these two categories were initially designated as reference oils where former group was assigned a viscosity index of 100 and the latter group as 0 (as demonstrated in the illustration below).

    Since then, this index has been largely popularized and proven to be highly useful for a range of professionals from manufacturer to a common user of lubricants. According the ASTM standard, viscosity index is:

    (Video) What is VISCOSITY INDEX? What does VISCOSITY INDEX mean? VISCOSITY INDEX meaning & definition

    —an arbitrary number used to characterize the variation of the kinematic viscosity of a petroleum product with temperature.

    Mathematically, viscosity index represents a relative measure of a lubricants temperature-viscosity behavior with respect to two reference oils. The index is estimated with the help of kinematic viscosity measured at two temperature points; therefore, unlike the empirical relationships, the index does not necessarily tell about the path that lubricants kinematic viscosity follows with respect to temperature. The illustration below shows a representation of the viscosity index.

    Viscosity Index Formula

    A stepwise procedure to estimate viscosity index, VI, may be outlined as below:

    • Measure kinematic viscosity of the unknown oil at Oil Viscosity Index and Viscosity Temperature Relation (25)C and Oil Viscosity Index and Viscosity Temperature Relation (26)C.
    • Get the look-up chart (ASTM D2270-86) of kinematic viscosity (for the information purposes only, the old chart is available here).
    • Identify two reference oils whose kinematic viscosity at Oil Viscosity Index and Viscosity Temperature Relation (27)C coincides with the kinematic viscosity of the unknown oil (from the look-up chart of ASTM standard).
    • Pick-up the kinematic viscosity of two identified reference oils at Oil Viscosity Index and Viscosity Temperature Relation (28)C
    • In reference to illustration above, now a, b, and c are known.
    • Use the following formula to calculate viscosity index, Oil Viscosity Index and Viscosity Temperature Relation (29) .
    Oil Viscosity Index and Viscosity Temperature Relation (30)

    It is important to mention here that a higher VI means the oils kinematic viscosity is less sensitive to temperature; therefore, the lubricant is likely to perform better over a wide range of temperature. On the other hand, in case of low VI, the viscosity of the lubricant may decline rapidly with respect to increase in temperature. Usually, synthetic oil based lubricants and multi-grade oils exhibit higher VI as compared to mineral oil based lubricants. Interestingly, the VI may go above 100 in case the value of Oil Viscosity Index and Viscosity Temperature Relation (31) in above figure appears in between Oil Viscosity Index and Viscosity Temperature Relation (32)and Oil Viscosity Index and Viscosity Temperature Relation (33). Usually, in industrial lubricant market, mineral oil based lubricants exhibit VI around 100; whereas, VI of synthetic oil based lubricant may go up to 150.

    It is important to know that a lubricants VI can be improved with selective additives in the lubricant formulation. High molecular weight polymers which are soluble such as polymethaacrylates above molecular weight of 10000 has found to be suitable for arresting viscosity declination with respect to temperature rise. These polymer additives are commonly being used for viscosity index improvement in multi-grade oils.

    In the process of lubricant selection, the VI of lubricants should not be ignored since optimum viscosity requirement of any system is often not precisely well known and machinery operating parameters such as load, speed, and temperature are likely to fluctuate. Low VI oil may be chosen when operating parameters of the machineries are almost constant (e.g. household applications); whereas, in a situation of highly fluctuating load and operating parameters (e.g. excavators, automobiles), high VI oil should be preferred. However, high VI oils incur high operational cost which in turn lead to a debate among industry professionals on the lubricants cost-VI trade-offs!

    Overall, the importance of understanding of the temperature-viscosity relationship is well recognized in industrial lubrication practices. While empirical relationships are useful for mathematical calculations and engineering design, viscosity index has been internationally accepted and popularized as a highly useful metric to determine the temperature-viscosity behavior of lubricants for practical purposes.

    Viscosity Index Calculator

    Here is a simple calculator to get the VI based on the kinematic viscosity values:

    (Video) Temperature Vs Oil Viscosity, Engine oil explained

    Classification of Viscosity Indexes of Oils

    Viscosity indexClassification
    Under 35Low
    Over 110Very High

    Video Explaining Viscosity Index

    About the Author

    Sandip Panda




    1. ASTM D341-03, 2003, Standard Practice for Viscosity-Temperature Charts for Liquid Petroleum Products, ASTM International, West Conshohocken, PA, 2017,astm.org
    2. Wright, A., 1969, An Improved Viscosity-Temperature Chart for Hydrocarbons, Journal of Materials, 4(1), pp. 19–27.
    3. Stachowiak, G. W. and Batchelor, W., 2000, Engineering Tribology (Second Edition), Butterworth-Heinemann, United Kingdom.
    4. Robinson, J. et al, Probing the molecular design of hyper-branched aryl polyesters towards lubricant applications, https://doi.org/10.1038/srep18624.


    What is the relation between temperature and viscosity in oil? ›

    A lubricant's viscosity will change with changes in temperature. As lubricants get hot, their viscosity drops; as they get cold, their viscosity increases. A viscosity index (VI) is assigned to a specific lubricant so that users have a clear understanding of the viscosity state at varying temperatures.

    How are viscosity and viscosity index related? ›

    Viscosity is a property which tends to decrease as the temperature increases. Viscosity index indicate the resistance of an oil to the changes in viscosity with temperature. smaller the change in viscosity with temperature, higher is the viscosity index of the oil.

    What is the effect of temperature on viscosity index? ›

    The lower the VI, the more the viscosity is affected by changes in temperature. The higher the VI, the more stable the viscosity remains over temperature fluctuations.

    What does viscosity index depend on? ›

    Viscosity-temperature behavior of oils. Compared to water, which has nearly the same flow behavior over a wide temperature range, oil changes its viscosity significantly with changing temperature. Additional influences such as oxidation, contamination, and pressure during operation have an impact on the viscosity.

    What is viscosity index in oil? ›

    The viscosity index (VI) of a lubricant is the rate of the viscosity change due to a temperature change. To know if a lubricant meets the asset's requirements based on the operating temperature range, you must understand the VI.

    How is oil viscosity calculated? ›

    It is calculated by dividing the dynamic viscosity by density (ρ), which is defined as mass per volume.

    How do you calculate viscosity index? ›

    The viscosity index according to DIN 2909 is calculated by comparing the change in viscosity of an oil with the two oils as a reference.
    Viscosity Index of lubricants
    1. All fields are required.
    2. V 40°C has to be greater than V 100°C.
    3. V 100°C has to be greater than 2 mm²/s.

    What does it mean if an oil has a high viscosity index rating? ›

    Oil viscosity index explained

    A higher viscosity implies a slower flow and a thicker fluid. The viscosity changes with temperature to ensure protection and operability under all circumstances. Thus, the viscosity index measures the ability of an engine oil to resist becoming thinner at high temperatures.

    How do you increase the viscosity index of lubricating oil? ›

    Conventionally, the lube oil stock obtained either by vacuum distillation or by deasphalting a vacuum residuum can be improved by solvent extraction to increase VI and by solvent dewaxing to lower the pour point.

    Why does viscosity increase with temperature? ›

    From the basis of the kinetic theory of gases, viscosity in gases increases with temperature because the increase in kinetic energy will increase the frequency of collisions between the molecules.

    Why does viscosity change with temperature? ›

    Increasing temperature results in a decrease in viscosity because a larger temperature means particles have greater thermal energy and are more easily able to overcome the attractive forces binding them together.

    How does temperature affect oil? ›

    Temperature & Viscosity

    In the heat, oil will become less viscous, or thinner. In the cold, it'll become more viscous, or thicker. This means that if you live in a very cold climate, you should be using thinner oil in your vehicle.

    What factors affect oil viscosity? ›

    The principal factors affecting viscosity are:
    • Oil composition.
    • Temperature.
    • Dissolved gas.
    • Pressure.
    4 Jun 2015

    What are the two major factors that affect viscosity? ›

    Those are:
    • Fluid Temperature: Usually the viscosity of liquids decreases with an increase in temperature. ...
    • Flow Conditions: For laminar flow the viscosity of liquid remains constant while for turbulent flow viscosity changes.
    • Pressure: With an increase in pressure, the viscosity of gases usually increase.
    12 Apr 2022

    What factors affect viscosity? ›

    CONCEPT: Viscosity: The state of being sticky, thick, and semi-fluid inconsistency, due to internal friction of fluids is called viscosity. The viscosity of a material is affected by temperature, pressure, nature of fluid, velocity gradient , etc.

    Why is viscosity index important? ›

    It allows for a better comparison of the viscosity behavior of different oils based on temperature. The viscosity index is important in order to ensure, e.g., the best possible lubrication for machinery, as temperature changes occur during operation.

    Is higher viscosity oil better? ›

    This is important, as engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils flow easier to protect engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures.

    What is the best viscosity for oil? ›

    For very cold weather, a viscosity rating of 5W-30 provides maximum protection. When driving in temperatures running from 0 to 60 degrees Fahrenheit, a rating of 10W-30 offers excellent all around protection. For driving in extremely hot conditions, 20W-50 oil offers the best protection.

    How can I calculate the viscosity index of an unknown oil? ›

    You need two reference oils. One with a VI of 100 and one with a VI of 0. Both must have the same viscosity as the unknown oil @ 100 C. The VI of the unknown oil is then: VI = 100(L-U)/(L-H).

    What is the unit of viscosity? ›

    What is the unit of viscosity? The unit of viscosity is newton-second per square metre, which is usually expressed as pascal-second in SI units.

    How can we determine the viscosity index of any sample oil? ›

    To determine an oil's VI, the viscosity is measured at two temperatures: 40 degrees C and 100 degrees C. This is then compared to a scale based on two reference oils. While the VI has no units of measure, it is well known that the viscosity index of a conventional mineral oil is between 95 to 100.

    What is low viscosity index? ›

    The greater the viscosity index (VI), the smaller the change in fluid viscosity for a given change in temperature, and vice versa. Thus, a fluid with a low viscosity index will experience a relatively large swing in viscosity as temperatures change. High-VI fluids, in contrast, are less affected by temperature changes.

    What does a negative viscosity index mean? ›

    A characteristic of a system in which momentum is transferred from a region of lower velocity toward a flow of higher velocity, increasing the mean wind shear.

    What does a higher viscosity number mean? ›

    In layman's terms, viscosity defines a fluid's resistance to flow. The higher the viscosity of a liquid, the thicker it is and the greater the resistance to flow.

    Which viscosity index is desirable? ›

    The higher the VI, the more stable the viscosity across a range of temperatures (more desirable). The temperatures used to determine the VI are 40 degrees C to 100 degrees C.

    What is the effect of temperature on viscosity of a liquid? ›

    Viscosity decreases on increasing the temperature of liquid because cohesive force among the water molecules decrease.

    Does hot oil get thicker? ›

    Your engine needs oil that is thin enough for cold starts, and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils.

    Why is lubricant oil high viscosity? ›

    Viscosity, or the thickness of the oil, is important because it determines the lubricant's film strength and efficiency in preventing friction between moving parts. Thick oil has a high viscosity, and thin oil has a low viscosity.

    Which additive is added to improve viscosity of an oil? ›

    The addition of an aftermarket additive, such as STP, generally thickens the oil. Such products can add about 10 viscosity points to an oil. So, a 10W-30 oil becomes 20W-40 oil.

    Which of the following indicates larger change in viscosity with temp? ›

    Explanation: The viscosity of an oil decreases with an increase of the temperature.

    What is the relationship between viscosity and temperature Brainly? ›

    Answer: In general, the viscosity of a simple liquid decreases with increasing temperature. As temperature increases, the average speed of the molecules in a liquid increases and the amount of time they spend "in contact" with their nearest neighbors decreases.

    Does viscosity increase with temperature? ›

    Viscosity generally decreases as the temperature increases. Viscosity generally increases as the temperature decreases. The viscosity of a liquid is related to the ease with which the molecules can move with respect to one another.

    Why does viscosity increase with temperature? ›

    From the basis of the kinetic theory of gases, viscosity in gases increases with temperature because the increase in kinetic energy will increase the frequency of collisions between the molecules.

    Why does viscosity decrease when temperature increases? ›

    The increase in temperature causes the kinetic or thermal energy to increase and the molecules become more mobile. The attractive binding energy is reduced and therefore the viscosity is reduced.

    What is true about the relationship between temperature and viscosity? ›

    Viscosity depends strongly on temperature. In liquids it usually decreases with increasing temperature, whereas, in most gases, viscosity increases with increasing temperature.

    What is the unit for viscosity? ›

    The SI unit of viscosity is the pascal second (Pa·s) or kg·m1·s1.

    What is the viscosity of cooking oil? ›

    Absolute Viscosities of Vegetable Oils at Different Temperatures and Shear Rate Range of 64.5 to 4835 s−1.

    What are the 4 factors that affect viscosity? ›

    The viscosity of a material is affected by temperature, pressure, nature of fluid, velocity gradient , etc.

    Does the viscosity change if the temperature become 30 degree? ›

    Viscosity of liquids decreases as the temperature rises.

    What is viscosity directly proportional to? ›

    In an ideal Newtonian fluid, the viscosity of the fluid is directly proportional to the shear rate of the fluid and the constant of proportionality is called the coefficient of viscosity.

    When viscosity increases what else increases? ›

    In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase). Increased viscosity increases the resistance to blood flow and thereby increases the work of the heart and impairs organ perfusion.

    What happens to the coefficient of viscosity if the temperature increases? ›

    As temperature increases, coefficient of viscosity decreases.

    How does viscosity depend on temperature and pressure? ›

    Solution : Viscosity of liquids decreases with increase of temperature. In case of water, viscosity decreases with increase of pressure. But in other liquids, viscosity increases with increase of pressure.

    How does temperature affect viscosity and flow rate? ›

    Increasing temperature will increase energy, and therefore the velocity of the particles. Viscosity will decrease with increased temperature because as particles move more quickly, they interact for shorter time (shorter interactions) reducing internal friction or stress and therefore decreasing viscosity.

    How does temperature affect the viscosity of liquid? ›

    The liquid viscosity is highly affected by the heat. The viscosity decreases with an increase in temperature.

    What is Newton law of viscosity? ›

    Newton's law of viscosity defines the relationship between the shear stress and shear rate of a fluid subjected to a mechanical stress. The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity.


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