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Viscosity classifications of motor oils

November 13, 2025
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Engine oil is vital for your car’s engine, and choosing the right viscosity is one of the most important decisions when maintaining your vehicle. But what exactly do the numbers and letters on an engine oil can mean? Why do some cars need 5W-30 while others require 10W-40? In this comprehensive article, we explore the scientific and practical aspects of engine oil viscosity classification.

Introduction: What is viscosity and why is it important?

Viscosity describes the resistance a liquid can exert to flow. In the case of motor oil, this property determines how effectively it protects engine parts at different temperatures.

Four reasons why the viscosity  of motor oil     is important:

  1. Corrosion protection at high operating temperatures

  2. Ability to operate at low temperatures (cold start)

  3. Formation of a sufficient protective layer between the parts

  4. Improved fuel consumption

SAE Classification System: Global Wife Language

The Society of Automotive Engineers (SAE) has established a global standard for classifying the viscosity of engine oils. This system defines two important oil properties:

  1. Low-temperature grease (number before W)

  2. High-temperature grease (number after W)

Understanding the meaning of the letter W in the classroom

The letter      W      stands for “Winter” and indicates the oil’s performance at low temperatures. The lower the number, the more suitable the oil is for cold climates.

  • 0W     : Suitable for extreme cold (down to -35 degrees Celsius)

  • 5W     : Good performance down to -30°C

  • 10W     : Suitable for temperate climates (down to -25 degrees Celsius)

  • 15 W     : for regions with mild winters (down to -20 °C)

  • 20W     : Primarily intended for warm weather.

Meaning of the second number: viscosity at operating temperature

The number after the letter W indicates the viscosity of the oil at engine operating temperature (100 degrees Celsius):

  • 20     : More fluid (suitable for modern engines with tight tolerances)

  • 30     : The most common features of modern cars

  • 40     : For older vehicles or warmer climates

  • 50-60     : for special cases or worn-out engines

Oil types according to viscosity

1. Seasonal oils (monoseasonal)

These oils have a uniform viscosity index (e.g., SAE 30 or SAE 40). They are used less frequently nowadays for the following reasons:

  • It becomes very viscous when cold.

  • It can become very thin in hot climates.

  • Only suitable for     older   engines or stable weather conditions.

2. All-season oils

These modern oils (e.g., 5W-30) contain:

  • High viscosity index (VI)     : low viscosity that varies with temperature.

  • Polymer reinforcement     : to achieve better performance at different temperatures.

  • All-round protection     : suitable for all seasons.

Viscosity selection table

Joe Temperature of the working environment Vehicle type
0W-20 From -35 °C to 20 °C New Japanese hybrid cars
5W-30 From -30 °C to 35 °C Most modern cars
10W-40 From -25 °C to 40 °C High-performance vehicles or older cars
15W-50 From -20 °C to 50 °C Sports cars, very hot areas

Factors that influence the selection of the appropriate viscosity

1. Recommendations of the vehicle manufacturer

I always follow the manufacturer’s recommendations, based on the following criteria:

  • Precision engine design

  • Distance between moving parts

  •    Lubrication system

They have already been identified.

2. Weather conditions

  • Cold regions     : Oils with a low W rating (0W or 5W)

  • Tropical regions     : second highest number (40 or 50)

  • For temperate climates     : 5W-30 or 10W-30 is ideal.

3. Age and condition of the engine

  • New engines     : thinner    oils   (5W-20, 0W-20)

  • High-performance engines     : thicker oils (10W-40)

  • Worn engines     : may require 15W-50

The influence of viscosity on engine performance

1. Cold start

Oils with a low W rating:

  • They flow faster.

  • Makes getting started easier

  • Reduces wear during start-up.

2. Fuel consumption

Soft oils:

  • This creates less resistance.

  • Reduces  fuel consumption   by up to 2%.

  • Ideal for hybrid vehicles

3. Protection against high temperatures

Oil number two:

  • Create a thicker protective layer.

  • Better suited for journeys with heavy loads.

  • It protects the engine better from overheating.

Common mistakes when choosing viscosity

  1. The use of very viscous oil in new engines     leads    to increased    fuel consumption and reduced efficiency.

  2. The use of very thin oil in older engines     may not provide sufficient protection.

  3. Changing the viscosity without consulting a mechanic     can lead to engine damage.

  4. Ignoring seasonal variations     :  Regions  with large temperature fluctuations may require seasonal viscosity adjustments.

Frequently Asked Questions

1. Can I use 10W-40 instead of 5W-30?

Answer: Only upon recommendation from the manufacturer or a qualified technician. This modification may affect vehicle performance and warranty coverage.

2. What is the difference between 5W-30 and 5W-40?

Answer: Both oils perform equally well in cold conditions, but 5W-40 is more viscous at high temperatures and is better suited for high-performance engines or tropical conditions.

3. Can low viscosity oil cause leaks?

Answer: No, this can happen with good engines, but it   can happen with worn engines.

Diploma

To   select the correct engine oil viscosity      , you must consider the following:

  • Requirements for a car engine

  • Weather conditions in the region

  • Construction recommendations

Always remember that      the ideal viscosity      is that of an oil which:

✓ It should be thin enough to flow easily in cold weather.
✓ It should be thick enough at operating temperature to form a protective layer.
✓ It should meet your engine’s specifications.

Choosing the right oil viscosity can extend the lifespan of your car engine and improve its performance.