The Impact of Temperature Fluctuations on Oil Viscosity
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The Impact of Temperature Fluctuations on Oil Viscosity

  • Katarina Knafelj Jakovac

    December 5, 2023

The basic physical characteristics of each lubricant include base oil, oil viscosity, viscosity index, and pour point, which is influenced by changes in temperature.

Characteristics of Base Oil

Every lubricating oil for machines and equipment consists of base oil and a certain amount of appropriate additives, depending on its specific purpose.

Mineral or non-synthetic base oils have a specific chemical composition primarily derived from purified oil by removing heavy paraffinic compounds, with further hydrogen refining.

In later steps of the production process, additives necessary to achieve targeted lubricant characteristics are blended, such as additives to improve viscosity and stability, prevent paraffin crystallization, or protect against corrosion.

In addition to naphthenic origin, mineral base oils can be paraffinic or aromatic, with the latter two groups having a higher viscosity index. On the other hand, naphthenic base oils are more prone to changes compared to paraffinic base oils.

Oils of paraffinic origin are preferred as raw materials due to their stability in blending with other components during lubricant production and for use in machines operating at higher temperatures.

Base oils of different origins, from left to right: mineral, synthetic, hydrocracked, synthetic with a high viscosity index, synthetic polyalphaolefin
Image: Base oils of different origins, from left to right: mineral, synthetic, hydrocracked, synthetic with a high viscosity index, synthetic polyalphaolefin (Source)

Additives in the base oil react to temperature changes and help maintain lubricant properties at the required level.

What is Viscosity?

The most important characteristic of any lubricating oil and the main determinant of classification is the ability to flow or internal friction that occurs during the flow of lubricant due to different speeds of movement of its layers.

It is measured in centistokes (cSt, 1 cSt = 0.01 St = 1 mm²/s= 10⁻⁶ m²/s) at 40°C and 100°C.

Oil viscosity is a major criterion for handling, storage, and operating conditions during application, making it mandatory information in the technical specifications of lubricant manufacturers, lubricant catalogs, and equipment manufacturer's user manuals.

The ASTM D445 standard prescribes testing methods for kinematic viscosity for transparent and opaque liquids and defines the method for calculating dynamic viscosity.

Kinematic viscosity of oil is determined by measuring the time it takes for a certain amount of oil to flow under the influence of gravity through a calibrated glass capillary viscometer.

Dynamic viscosity (mPa·s) is obtained by multiplying the value of kinematic viscosity by density.

For example, when an oil film forms between a bearing and a shaft, some oil molecules adhere to the metal surface of the shaft, while others accumulate on the surface of the bearing, creating resistance to stress.

This behavior of oil is influenced by temperature. Multigrade oils of lower viscosity have higher stress resistance, while single-grade oils have lower stress resistance.

Lubricating oils of lower viscosity and potentially higher stress resistance must maintain a sufficient thickness of the oil film; otherwise, an increase in temperature can lead to a reduction or disappearance of the oil film and friction between two metal surfaces, which should be avoided.

If contact between two metal surfaces does occur, it is said to be a seizure.

In Figure 1, we can see how the viscosity of oil behaves depending on the rise or fall of the operating temperature for a specific VG grade.

Change in viscosity depending on temperature
Image: Change in viscosity depending on temperature (Source)

If the viscosity of the oil is too high with potentially low stress resistance, the internal friction will drastically increase the temperature of the oil itself, leading to overheating and oxidation of the oil.

For this reason, it is crucial to consider the operating temperature throughout the machine's life when selecting the appropriate lubricant.

What is the Pour Point of Oil?

The pour point is defined as the lowest temperature at which lubricating oil has the ability to flow.

It is often mistakenly considered a criterion for selecting the viscosity grade VG.

For example, if the oil has a pour point of -30°C, most people will automatically assume that the oil will spread over the surface of the machine part even when the external temperature is -30°C.

And they would be wrong.

In the best case, when oil with a pour point at -30°C lubricates a machine in an environment where the temperature is -30°C, there will be splashing, leading to an increase in the oil temperature due to the internal movement of layers.

The temperature increase allows a decrease in viscosity just enough for the oil to start flowing slowly over machine parts and grooves.

Often, this process takes 5 to 10 minutes (sometimes more), during which the machine operates with unlubricated components, which can lead to seizure because the oil has become too thick to properly lubricate.

The point of the described situation is: do not rely on the pour point as the main criterion when choosing the viscosity grade of lubricating oil.

What is Viscosity Index?

Viscosity Index (abbreviated as VI or IV in English, referring to the viscosity index) describes the resistance to changes in viscosity depending on temperature changes.

The image illustrates the change in kinematic viscosity with temperature for motor, synthetic, and mineral oils, with mineral oil having a low viscosity index.

Promjena indeksa viskoznosti ovisno o temperaturi i vrsti ulja.png
Image: Change in viscosity index with temperature and oil type

To enhance viscosity resistance to temperature changes, it is necessary to improve the manufacturing process or add additives to motor oil.

When observing an oil film whose thickness decreases with an increase in temperature, we say that the oil has a low viscosity index.

If the oil viscosity remains relatively unchanged when heated, it has a higher viscosity index and is considered more stable.

The relationship between temperature and viscosity is crucial when selecting oil for lubricating a machine with a drastically changing operating temperature, especially in northern latitudes or arctic regions.

All the aforementioned characteristics must be specified by lubricating oil manufacturers in the technical specifications of each lubricant, according to the requirements of standards ISO 2909:2002 or ASTM D2270.

In a table extracted from such technical specifications, the characteristics are highlighted in yellow. The first column lists the characteristic, the second column indicates the standard under which each characteristic was tested in laboratory conditions.

The third column expresses the units of measurement, and the fourth column provides the measured value.


For example, the viscosity index of motor oil was tested according to the ASTM D2270 standard, has no unit of measurement, and is 164.

Machine manufacturers often fail to mention the viscosity index in the lubrication and maintenance chapters of user manuals, so it is advisable to check compatibility before using a specific lubricant.

Maintenance personnel and other users of motor oils rarely consult lubricant manufacturers for expert assistance in selecting lubricants based on defined needs, technical specifications, or specific working conditions.

In my experience, lubricant manufacturers are very willing to answer specific questions, provide technical data on oils, offer guidance on handling waste oils, participate in creating lubrication plans, and provide instructions and recommendations for lubricant application.

Some manufacturers are even willing to organize short educational sessions and workshops on lubricants and the fundamentals of lubrication.

I recommend reaching out to your supplier or the manufacturer of the lubricating oil used in your machines and equipment to inquire about education for you and your professional team. Be sure to ask anything you need to know about their lubricants if you haven't done so before.

If you have encountered issues with a particular type of lubricant leading to machine failure, always provide feedback to the supplier or manufacturer.

This way, the manufacturer gains insight into the behavior of their product in real conditions, and you receive assistance in investigating the cause of the failure.

How do we calculate the oil viscosity index?

The viscosity index for a selected oil is calculated according to the ASTM D2270 standard for measured viscosity at temperatures of 40°C and 100°C.

Standard paraffinic oil has a viscosity index VI=100, while standard naphthenic oil has a viscosity index VI=0.

First, measure the oil viscosity at 100°C, and then extract parameters from the graph in the ASTM D2270 standard:
L – viscosity of naphthenic-aromatic structure oil at 40°C
H – viscosity of paraffinic structure oil at 40°C
U – viscosity of the tested oil at 40°C \

All these parameters are related by the formula:

IV=LULH×100IV = \frac{L-U}{L-H}\times 100

The relationships between these parameters are graphically represented on a diagram, observing the temperature range between 20°C and 100°C.

Image: Relationship between viscosity of naphthenic-aromatic and paraffinic structure oils
Image: Relationship between viscosity of naphthenic-aromatic and paraffinic structure oils (Source)
A quicker way to calculate the viscosity of oil using the described formula is by utilizing an online calculator or using a mobile application.

Oil Viscosity in Practical Examples

New pumps are delivered with empty bearing housings, and before starting the pump for the first time, it is mandatory to fill the bearing housing with the quantity of lubricating oil and viscosity grades prescribed by the pump manufacturer.

It is not allowed to mix oils of different viscosity grades as doing so would cause more harm than good.

For example, for a standard single-stage centrifugal pump with a power of 50 kW, basic characteristics for lubricating oil under working conditions ranging from -10 °C to 40 °C are defined. The maximum operating temperature of the bearings is 110 °C.

The oil must have a mineral or synthetic base, a minimum viscosity index VI min: 95, viscosity grade ISO VG 100, working temperature of the oil is from -10 °C to + 170 °C, flash point > 200 °C.

The complete oil change interval ranges from 1000 to 1500 hours of operation, and the replacement must be performed at least once a year.

Cross-section of a single-stage centrifugal pump
Image: Cross-section of a single-stage centrifugal pump (Source)

The basic procedure for oil replacement in the bearing housing of a single-stage centrifugal pump includes the following steps:

  1. Remove the plug at position 1 on the bearing housing and pour the prescribed oil using a funnel.
  2. Check on the sight glass if the quantity of poured oil into the bearing housing has reached the mark on the sight glass indicated at position 4 in the image. The oil quantity is sufficient for continuous pump operation.

Oil replenishment is done by lifting and tilting the oil cup at position 2, as shown in the image:

Cross-section of the bearing housing and marked steps of the oil replacement procedure
Image: Cross-section of the bearing housing and marked steps of the oil replacement procedure (Source)

Then, the oil cup is returned to its initial position, ensuring that the oil level is sufficient for the ball bearing at position 3 to be immersed to the height of the central line.

The oil quantity in the bearing housing gradually decreases during pump operation, so it is necessary to regularly check the level on the sight glass and perform oil top-ups regularly.

It should be noted that the viscosity of used oil and newly added oil differs when mixed in the bearing housing.

Used oil is older, may contain traces of dirt particles, be oxidized, or have sediment, affecting viscosity.

For this reason, regular laboratory analyses should be conducted to monitor the oil condition, especially for powerful machines like turbo compressors, gas turbines, or diesel generators.

When it comes to automobiles, recent research has enabled the production of motor oils with a higher viscosity index that undergoes minimal changes when the engine operates, contributing to fuel efficiency.

Every automobile engine is designed to be lubricated with oil whose viscosity must be within certain values to reduce friction and prevent part wear.

The less the viscosity of a specific motor oil changes due to temperature variations, the higher the potential for using that motor oil in both low and high ambient temperature conditions.

When the motor oil is cold (either because it's winter outside or because the engine is just starting from rest), its viscosity is low. It doesn't reach the lubrication points quickly enough, which can lead to damage on surfaces due to friction.

ACEA (Association des Constructeurs Europeens de I’Automobile) is an association of European car manufacturers that have set criteria for determining the quality of motor oil. Member companies of the association include all European automobile manufacturers.

The technical specifications of motor oil provided by the manufacturer list high-performance automobile brands using the oil, compliance with ACEA criterion C3 for high-torque stability oil (staying in grade), compatibility with diesel exhaust particulate filter, and compatibility with the catalyst.

Pouring oil into a car engine
Image: Pouring oil into a car engine (Source)

Tests of oil effectiveness, to which the lubricating oil was subjected to gain approval for market placement, are also mentioned. In this case, the oil was tested on Porsche and VW engines.

Extract from the technical specifications of the motor oil manufacturer
Image: Extract from the technical specifications of the motor oil manufacturer (Source)

It is interesting to note that certain automobile brands like Porsche produce their own motor oil and recommend its use in their models.

Porsche motor oil
Image: Porsche motor oil (Source)

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
December 5, 2023

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.