What You Need to Know – Part 3

Understanding Engine Oil Viscosity

In Part 2, we discussed the important functions that engine oil must perform in modern engines. In this section, we will focus on one of the most important properties of engine oil: viscosity.

Viscosity plays a critical role in engine lubrication because it determines how easily oil flows through the engine and how effectively it maintains a protective oil film between moving components.

What Is Viscosity?

Viscosity refers to an oil’s resistance to flow. It is one of the most important properties used to describe and classify engine oils.

Engine oil viscosity changes with temperature:

• At low temperatures, oil becomes thicker and flows more slowly.

• At high temperatures, oil becomes thinner and flows more easily.

Engine oil must be able to flow sufficiently during cold starts while also maintaining enough thickness at normal operating temperatures to provide effective lubrication and component protection.

For this reason, modern engine oils are formulated to maintain suitable viscosity across a wide range of operating temperatures.

Viscosity Index

Because oil viscosity changes with temperature, a measurement is needed to describe how significantly its viscosity changes.

This measurement is known as the Viscosity Index (VI).

Viscosity Index is traditionally determined by comparing an oil’s viscosity at 40°C and 100°C. A higher VI generally indicates that the oil experiences a smaller change in viscosity as temperature changes.

This property is particularly important for engines that operate across a wide range of temperatures.

Many synthetic and highly refined engine oils can provide a higher viscosity index than conventional mineral oils, depending on their formulation and base-oil technology.

SAE Viscosity Grades

The Society of Automotive Engineers (SAE) developed a classification system for defining the viscosity grades of engine oils.

Engine oil viscosity grades are specified under SAE J300, while automotive gear lubricants are classified under SAE J306.

The SAE system provides standardized viscosity requirements for different operating conditions, including:

• Cold-start performance

• Oil pumpability

• High-temperature viscosity

• Operating-temperature lubrication

This allows vehicle manufacturers and lubricant users to select an oil with an appropriate viscosity for a particular engine and operating environment.

What Is a Multigrade Engine Oil?

Most modern automotive engine oils are multigrade oils. A multigrade oil is designed to meet viscosity requirements at both low and high temperatures.

For example, a 5W-30 engine oil has:

• 5W – Represents its low-temperature viscosity classification.

• 30 – Represents its high-temperature viscosity classification.

The “W” stands for Winter, indicating that the first number relates to the oil’s low-temperature performance.

Multigrade oils provide a balance between cold-start flow and high-temperature protection.

Understanding the Numbers Before and After “W”

The number before the W indicates the oil’s low-temperature viscosity performance.

Generally:

• A lower W number indicates better low-temperature flow characteristics.

• A lower W grade can help the oil circulate more easily during cold starts, provided it is appropriate for the engine.

The number after the W represents the oil’s high-temperature viscosity classification.

Generally:

• A higher second number indicates a higher viscosity at the SAE high-temperature test conditions.

• The appropriate high-temperature grade helps maintain the required oil film and lubrication under operating conditions.

However, a higher viscosity grade is not automatically better. The correct viscosity must always be selected according to the vehicle manufacturer’s requirements.

High-Temperature / High-Shear Viscosity

Engine oil also needs to maintain suitable viscosity under severe operating conditions.

High-Temperature / High-Shear (HTHS) viscosity evaluates the behavior of engine oil under high temperature and high shear conditions that can occur in areas such as:

• Engine bearings

• Camshafts

• Piston rings

• Other highly loaded engine components

HTHS viscosity provides an indication of the oil’s resistance to flow under severe conditions and its ability to maintain an effective lubricating film.

An oil that becomes excessively thin under high-temperature and high-shear conditions may not provide the level of protection required by certain engines.

Viscosity Measurement Units

Viscosity can be expressed using different measurement units depending on the test method.

Common units include:

• Centistokes (cSt) – Commonly used for kinematic viscosity measurements.

• Centipoise (cP) – Used for dynamic viscosity measurements.

These measurements are used in different standardized tests to evaluate lubricant flow characteristics under specific conditions.

Understanding 5W-30 Engine Oil

A 5W-30 engine oil is a common example of a multigrade lubricant.

The 5W indicates its low-temperature viscosity classification, while 30 represents its high-temperature viscosity classification.

As temperature changes, the oil’s actual viscosity also changes. The SAE classification confirms that the oil meets defined viscosity requirements at specified test conditions.

This allows a 5W-30 oil to provide suitable cold-temperature flow while maintaining the required viscosity at operating temperature.

Why Multigrade Oils Are Important

The majority of modern automotive engine oils are multigrade because engines operate across a wide range of temperatures.

A multigrade formulation is designed to provide:

• Good cold-start flow

• Effective oil circulation

• Reliable lubrication during normal operation

• Appropriate high-temperature viscosity

• Consistent protection across changing operating conditions

This combination helps provide effective lubrication from engine startup through normal operating temperatures.

Choosing the Correct Oil Viscosity

With many viscosity grades available, selecting the correct engine oil can sometimes be confusing.

The best approach is to always follow the vehicle manufacturer’s recommendations.

The owner’s manual or official service information normally specifies the required:

• SAE viscosity grade

• API performance category

• ILSAC specification

• ACEA specification

• Manufacturer-specific approvals

Some vehicles may permit different viscosity grades depending on ambient temperature, engine design, operating conditions, or service requirements.

Viscosity and Operating Conditions

Engine oil viscosity should be suitable for the conditions in which the vehicle operates.

Factors that can influence oil selection include:

• Ambient temperature

• Engine design

• Vehicle age

• Driving conditions

• Engine load

• Towing requirements

• Stop-and-go traffic

• High-speed operation

• Manufacturer specifications

Using an oil that is too thick or too thin for a particular engine may affect lubrication, oil circulation, fuel efficiency, and engine protection.

Ignitol Engine Oils

Ignitol Engine Oils are available in a range of viscosity grades designed to meet the lubrication requirements of different engines and operating conditions.

Selecting the appropriate Ignitol engine oil according to the recommended SAE viscosity grade and required performance specifications can help support effective lubrication, engine protection, and reliable performance.

To Conclude

Viscosity is one of the most important properties of engine oil because it directly influences oil flow, lubrication, and engine protection across different temperatures.

Understanding SAE viscosity grades such as 0W-20, 5W-30, 10W-40, and 15W-40 can help vehicle owners better understand engine oil selection.

However, the correct viscosity is not simply the thickest or thinnest available option. Always follow the vehicle manufacturer’s recommended SAE viscosity grade and performance specifications to ensure the engine receives the appropriate level of lubrication and protection.