Lubricant Viscosity Explained: How to Choose Machine Oil

Is thicker oil always better for machinery? Why can an oil that appears suitable at room temperature become ineffective when a machine becomes hot? Lubricant viscosity is one of the most important properties in oil selection, but it is also one of the most commonly misunderstood.

Lubricant viscosity is a measure of a fluid’s resistance to flow or shear. Machine oil must be viscous enough at operating temperature to maintain a separating film, but fluid enough to circulate, reach the contact and allow efficient start-up. The correct grade depends on the equipment manufacturer’s specification, operating temperature, speed, load and lubrication system.

This guide explains viscosity, viscosity grades, temperature effects and the practical steps involved in selecting machine oil.

What is lubricant viscosity?

Viscosity describes how strongly a fluid resists deformation and flow. A high-viscosity liquid resists movement more than a low-viscosity liquid under the same conditions.

Comparing syrup with water provides a simple visual analogy, but lubricant selection is more complex. A thicker oil is not automatically a better oil. It may support a thicker film in one contact, yet circulate too slowly, create excess drag or fail to reach a component during start-up.

Two forms of viscosity are commonly encountered in technical data.

Dynamic viscosity

Dynamic viscosity describes resistance to shear between adjacent layers of fluid. Its SI unit is the pascal-second (Pa·s), although lubricant data may also use the millipascal-second (mPa·s) or centipoise (cP).

Dynamic viscosity is particularly relevant where the force required to move or pump the fluid matters. Low-temperature engine-oil tests, for example, assess whether an oil remains sufficiently mobile for starting and pumping.

Kinematic viscosity

Kinematic viscosity relates dynamic viscosity to the fluid’s density. Its SI unit is square metres per second, but lubricant data commonly use square millimetres per second (mm²/s), which is numerically equivalent to the centistoke (cSt).

Industrial oil grades are normally associated with kinematic viscosity. ASTM D445 is the standard test method used to determine kinematic viscosity for transparent and opaque liquids and to calculate dynamic viscosity where density is known.[1]

Why is viscosity important in a machine?

Machine surfaces are not perfectly smooth. At a microscopic scale, their peaks can interact when components slide or roll against one another. A lubricant must establish a film that separates those surfaces sufficiently for the intended operating regime.

If viscosity is too low at the contact, the oil film may become inadequate and the risk of surface interaction, heat and wear can increase. If viscosity is too high, the machine may experience excessive fluid friction, slow circulation, poor cold starting or unnecessary energy loss.

Viscosity therefore involves a balance:

• Film formation: enough viscosity to support separation under the applied load.
• Circulation: enough fluidity to pass through pumps, lines, filters and clearances.
• Start-up: acceptable flow before the machine reaches normal temperature.
• Heat control: avoiding unnecessary churning and viscous resistance.
• Delivery: reaching every required contact in the correct quantity.

For a broader introduction to lubrication functions, see iLearn Engineering’s guide to lubricants and lubrication systems in machinery.

How does temperature affect oil viscosity?

Liquid lubricants generally become less viscous as their temperature rises and more viscous as their temperature falls.

This means that room-temperature appearance is a poor basis for selection. The important question is whether the oil has suitable viscosity at the machine’s actual operating temperature and remains sufficiently fluid during its coldest expected start.

When oil is cold

At low temperatures, oil can become difficult to pump or distribute. A machine may experience:

• Slow oil delivery after start-up
• Increased starting torque
• Pressure drops across filters or restrictions
• Temporary starvation at remote lubrication points
• High viscous drag

When oil is hot

As temperature rises, viscosity decreases. If the oil becomes too thin for the contact, it may provide insufficient film thickness. Leakage can also increase, and an oil pump may deliver less effective pressure or flow depending on the system design.

This is why bearing-oil selection is based on viscosity at operating temperature rather than the nominal grade alone. SKF’s guidance relates the required viscosity to bearing size, rotational speed and operating temperature.[2]

What is viscosity index?

Viscosity index, usually abbreviated to VI, indicates how an oil’s kinematic viscosity changes with temperature. A higher viscosity index generally means that the oil changes viscosity less, relative to a lower-VI oil, over the temperatures used in the calculation.

ASTM D2270 calculates viscosity index from kinematic-viscosity measurements at 40°C and 100°C.[3]

Viscosity index should not be confused with viscosity grade:

• Viscosity grade identifies a viscosity range under the conditions defined by a classification system.
• Viscosity index indicates the oil’s relative viscosity–temperature behaviour.
• Neither value, by itself, describes every aspect of lubricant performance.

An oil with a high VI may be helpful where operating temperatures vary widely. It must still meet the required viscosity, additive, material-compatibility and equipment specifications.

What does ISO VG mean?

ISO VG stands for International Organization for Standardization Viscosity Grade. ISO 3448 establishes the viscosity-classification system for industrial liquid lubricants.[4]

An ISO VG number is centred on the oil’s kinematic viscosity in mm²/s at 40°C. For example, an ISO VG 68 oil has a nominal kinematic viscosity of approximately 68 mm²/s at 40°C, within the tolerance defined by the standard.

Common industrial grades include:

• ISO VG 32
• ISO VG 46
• ISO VG 68
• ISO VG 100
• ISO VG 150
• ISO VG 220
• ISO VG 320
• ISO VG 460

These numbers describe viscosity classes rather than overall quality or suitability.

Two oils labelled ISO VG 68 may have different:

• Base-oil types
• Viscosity indices
• Additive packages
• Load-carrying properties
• Resistance to oxidation, water or foaming
• Seal and material compatibility
• Equipment approvals

ISO VG is therefore only one part of an industrial-lubricant specification.

What is an SAE viscosity grade?

SAE viscosity grades are commonly associated with engine and automotive drivetrain oils. SAE J300 defines the rheological limits used to classify engine lubricating oils; it does not classify the oil’s other performance characteristics.[5]

An engine-oil designation such as 10W-40 contains two grade references:

• The grade containing W relates to specified low-temperature cranking and pumping performance.
• The second grade relates to viscosity requirements at higher test temperatures.

It is misleading to say that 10W-40 simply “behaves like a 10 when cold and a 40 when hot”. The designation means the oil satisfies defined low- and high-temperature rheological limits within SAE J300.

The letter W is associated with winter performance; it does not mean weight.

ISO VG vs SAE: what is the difference?

ISO VG and SAE grades belong to different classification systems and should not be treated as directly interchangeable.

Feature	ISO VG	SAE J300
Main use	Industrial liquid lubricants	Engine lubricating oils
Classification basis	Kinematic viscosity centred on a grade at 40°C	Defined low- and high-temperature rheological limits
Example	ISO VG 68	SAE 10W-40
Does the grade define full performance?	No	No
Can the numbers be compared directly?	No	No

If a gearbox specifies ISO VG 220, an SAE engine oil should not be selected merely because a conversion chart suggests a broadly similar viscosity. The required additives, approvals and operating characteristics may be completely different.

What are monograde and multigrade oils?

A monograde oil satisfies one viscosity grade within the relevant classification system. A multigrade engine oil satisfies both a low-temperature W grade and a higher-temperature grade under SAE J300.

Multigrade performance can be achieved through suitable base oils, viscosity-index improvers or a combination of formulation approaches. Shell explains that viscosity-index improvers reduce the rate at which some oils lose viscosity as temperature rises, although such polymers can also be affected by shear.[6]

The purpose is not to prevent viscosity changing altogether. All practical liquid lubricants change viscosity with temperature. The objective is to manage that change so the oil remains pumpable when cold and sufficiently viscous under the specified higher-temperature conditions.

Multigrade engine-oil terminology should not be applied casually to industrial oils. Industrial products are normally selected using the equipment specification, ISO VG class and relevant performance requirements.

What happens if machine oil is too thick?

Oil with excessive viscosity for the application can cause:

• Slow circulation or delayed arrival at the contact
• High starting torque
• Increased viscous drag and energy consumption
• Excessive heat generation
• Poor filter flow or filter bypass
• Inadequate splash distribution in a gearbox
• Cavitation or restricted inlet flow at a pump

A high pressure reading does not necessarily mean that lubrication is effective. Pressure can rise because the oil is resisting flow while a distant component receives insufficient supply.

What happens if machine oil is too thin?

Oil with insufficient viscosity at operating temperature can contribute to:

• Inadequate film thickness
• Increased surface interaction and wear
• Leakage through seals and clearances
• Reduced damping of shock or vibration
• Lower effective pressure in some lubrication systems
• Increased oil consumption

These symptoms can also have other causes. A viscosity change should not be made until operating conditions, equipment condition and manufacturer guidance have been reviewed.

How do speed and load influence viscosity selection?

As a general tendency, slow and heavily loaded contacts may require a more viscous oil than fast, lightly loaded contacts because the lubricant has more difficulty maintaining separation under high load and low entrainment speed.

High-speed contacts can draw oil into the contact effectively, but excessive viscosity may create drag and heat. This does not mean that every fast machine needs a low-viscosity oil or every heavily loaded machine needs the thickest available product. Geometry, surface finish, temperature, lubricant supply and motion type all affect film formation.

Use calculation methods and selection charts from the equipment or component manufacturer wherever available.

A practical VITAL viscosity checklist

The following VITAL checklist is an iLearn Engineering framework for organising an oil-viscosity decision. It is not an industry standard and does not replace manufacturer guidance.

• V — Verify the specification: Identify the required viscosity grade, lubricant type, approvals and performance category.
• I — Identify temperatures: Record the lowest start-up temperature, normal oil temperature and credible peak temperature.
• T — Think about speed and load: Consider rotational or sliding speed, load, shock, starts, stops and duty cycle.
• A — Assess the lubrication system: Check pumps, filters, pipe sizes, splash paths, reservoirs and component clearances.
• L — Look at the complete data: Review viscosity at relevant temperatures, VI, pour point, approvals, additive system and compatibility—not just the grade printed on the label.

If the manufacturer’s requirement is unavailable or operating conditions have changed substantially, obtain engineering or lubricant-supplier advice before substituting a grade.

Three illustrative selection scenarios

These examples demonstrate the reasoning process. They are not product recommendations.

Scenario 1: Industrial gearbox running hotter than expected

An enclosed gearbox specifies ISO VG 220, but its oil temperature has increased after a rise in production duty.

Changing immediately to a thicker grade could hide rather than solve the problem. The cause of the higher temperature should first be investigated: overload, misalignment, contamination, incorrect oil level, restricted cooling or mechanical damage may be involved.

The oil’s viscosity at the new operating temperature and the manufacturer’s permitted grade range can then be assessed.

Scenario 2: Bearing that is difficult to start in winter

A circulating-oil bearing receives oil slowly during very cold starts.

The decision should consider the specified operating viscosity, minimum start-up temperature, pumpability, line and filter restrictions, heater arrangements and whether a higher-VI product is approved.

Selecting a lower ISO VG grade without checking hot-running film requirements could create a different problem after warm-up.

Scenario 3: Engine oil labelled 10W-40

The user assumes the oil is equivalent to ISO VG 40 because both labels contain the number 40.

This is incorrect. SAE 10W-40 and ISO VG 40 belong to different systems, use different test requirements and serve different applications. The required engine specification, viscosity grade and manufacturer approval should be followed.

Common viscosity-selection mistakes

Choosing oil by appearance

An oil’s apparent thickness when poured at room temperature does not establish its viscosity at operating temperature or its suitability for a machine.

Assuming a higher grade gives more protection

Higher viscosity may improve film thickness in some circumstances, but it can also reduce circulation and increase drag, heat and start-up difficulty.

Treating ISO VG and SAE numbers as equivalent

The systems have different purposes and test conditions. Similar numbers do not establish equivalence.

Ignoring the operating temperature

The viscosity that matters is the viscosity in the working contact. A nominal grade measured at a reference temperature cannot be interpreted without considering the machine’s real temperature.

Looking only at viscosity

Correct viscosity does not guarantee that an oil has the correct additives, approvals, base oil or compatibility. A hydraulic fluid, turbine oil, gear oil and engine oil can have similar kinematic viscosities but very different formulations.

If the first decision is which lubricant form suits the application, use iLearn Engineering’s comparison of oil, grease and dry lubricants.

Using flash point as an operating-temperature limit

Flash point concerns the formation of ignitable vapour under specified test conditions. It is not a recommended continuous oil temperature and does not replace the lubricant manufacturer’s operating limits.

Develop your understanding of machines and mechanical systems

Understanding oil viscosity supports the wider study of bearings, gears, hydraulic components, seals, power transmission and maintenance.

The Diploma in Mechanical Technology is a focused 40-credit Level 4 route covering engineering science and machines and systems.

For broader study, the Higher International Certificate in Mechanical Engineering provides a 120-credit Level 4 foundation.

The Higher International Diploma in Mechanical Engineering comprises 240 credits in total, incorporating Level 4, while the International Graduate Diploma in Mechanical Engineering comprises 360 credits in total and incorporates Level 5.

Compare these mechanical engineering pathways and select the depth of study that best supports your professional-development objectives.

Frequently asked questions

Is thicker oil better for machinery?

Not necessarily. Oil must be viscous enough at operating temperature to support the required film but fluid enough to circulate, pass through filters and reach components during start-up.

What unit is used for oil viscosity?

Kinematic viscosity is commonly reported in mm²/s or centistokes. Dynamic viscosity may be reported in Pa·s, mPa·s or centipoise.

What does ISO VG 68 mean?

ISO VG 68 identifies an industrial-oil viscosity grade centred on a kinematic viscosity of approximately 68 mm²/s at 40°C. It does not describe the oil’s complete performance or application suitability.

Is ISO VG 46 the same as SAE 46?

No. ISO VG and SAE grades use different classification systems and test conditions. A product must be selected using the system and specification required by the equipment manufacturer.

What does the W in 10W-40 mean?

The W identifies the low-temperature or winter portion of the SAE J300 grade. It does not mean weight.

Does oil get thicker when it is hot?

Normal liquid lubricants become less viscous as temperature rises. Multigrade formulations reduce the rate of viscosity loss across their intended range; they do not reverse the fundamental relationship.

What is a high-viscosity-index oil?

A high-VI oil changes viscosity less, relative to a lower-VI oil, across the temperatures used in the VI calculation. It must still meet the required viscosity grade and equipment specification.

Can the wrong viscosity damage a machine?

Yes. Oil that is too thin can provide inadequate film thickness, while oil that is too thick can restrict flow, increase drag and create start-up problems. Equipment condition and operating factors should also be investigated before attributing a fault solely to viscosity.

References

[1] ASTM International: ASTM D445—Kinematic viscosity of transparent and opaque liquids

[2] SKF: Selecting a suitable oil

[3] ASTM International: ASTM D2270—Calculating viscosity index

[4] International Organization for Standardization: ISO 3448 industrial-liquid-lubricant viscosity classification

[5] SAE International: SAE J300 engine-oil viscosity classification

[6] Shell: Multigrade oils explained

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