What Does Motor Oil Viscosity Mean?

The viscosity of a fluid indicates its resistance to flow. When external forces such as gravity act on a liquid, its molecules move against each other, creating internal friction resistance. The greater this friction, the higher the viscosity of the liquid.

Simply put, viscosity describes how thick or fluid a substance is. The classification of lubricant viscosity is typically based on kinematic and dynamic measurement systems, established by standards such as those from the Society of Automotive Engineers (SAE) or the International Organization for Standardization (ISO).

The Effect of Temperature on Viscosity

The response of viscosity to temperature and pressure fluctuations determines how well a lubricant can perform its essential functions. Base oils thicken and solidify at lower temperatures when the so-called pour point is exceeded. This increases the load-carrying capacity of the lubricant but significantly worsens its circulation ability. As the temperature rises, lubricants become thinner, reducing load-carrying capacity while improving flowability.

The Effect of Pressure on Viscosity

Extreme pressure between surfaces can reduce viscosity due to mechanical shear. This weakens the lubricant film, which is designed to prevent metal-to-metal contact and wear. While a higher viscosity could compensate for this, it would also impair the lubricant’s flowability and cooling performance. Choosing the correct viscosity is therefore crucial to ensuring the necessary oil flow and stable lubrication under pressure.

Two-Stage Motor Oil Viscosity

The viscosity of multi-grade motor oils is indicated by a combination of two numbers and a letter, such as 5W-30, 5W-40, or 0W-30. This designation is important for drivers as it often forms the basis for selecting the right motor oil.

The first number with the letter W (e.g., 0W) describes the cold start viscosity of the motor oil, where W stands for "Winter".
The second number (e.g., 30) represents the oil's viscosity at normal operating temperature (100°C).

Kinematic Viscosity

This method is used to measure the second number (e.g., 30). It describes the viscosity of the motor oil at operating temperature and is expressed in centistokes (cSt) or mm²/s. This value determines the viscosity class of the motor oil.

Dynamic or Absolute Viscosity

This method measures the first number (e.g., 0W). The dynamic viscosity is expressed in centipoise (cP) or millipascal-seconds (mPa.s) and is used to determine the cold start performance of motor oil. Since 65-75% of engine wear occurs during cold starts, this value is particularly important.

To determine the cold start capability of a motor oil, two tests are performed:

Cold Cranking Test: Measures whether the motor oil remains fluid enough at low temperatures to allow the starter to turn.
Pumpability Test: Determines whether the oil stays fluid enough to be transported by the oil pump.
For a motor oil to qualify as 0W, it must pass the Cold Cranking Test at -35°C and the Pumpability Test at -40°C.

SAE Viscosity Classification – Cold Startability – Cold Pumpability

0W: 6,200 @ -35°C – 60,000 @ -40°C
5W: 6,600 @ -30°C – 60,000 @ -35°C
10W: 7,000 @ -25°C – 60,000 @ -30°C
15W: 7,000 @ -20°C – 60,000 @ -25°C
20W: 9,500 @ -15°C – 60,000 @ -20°C

A 0W viscosity must meet stricter low-temperature conditions than a 20W viscosity, which was more common in the 1970s.

HTHS Viscosity

HTHS viscosity (High Temperature High Shear) describes the resistance of a lubricant to flow under extreme high temperatures (150°C) and constant shear. This test simulates hot engine conditions with tight tolerances and high speeds between moving engine parts.

A lower HTHS value means higher fuel efficiency
A higher HTHS value means better wear protection
Modern engines require only motor oils with low HTHS viscosity, as these increase fuel efficiency and reduce emissions.

Conclusion

The right viscosity of motor oil depends on the bearing clearances and the design of the engine's oil system. Since motor oil has a significant impact on engine performance, choosing the right motor oil is essential.