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?What is the viscosity grade of a motor oil

5W30, 15W40, 10W40… Not always easy to understand and translate these indications when choosing a bottle of motor oil. In fact, these figures represent the oil’s viscosity grade, which is the lubricant’s level of fluidity and its efficiency at low and high temperatures. Let’s look at the various types of grades and their meaning.


The viscosity grade of a motor oil provides information on the oil’s resistance to flow in your vehicle’s engine. A lubricant with a low viscosity grade will be more fluid, more liquid, and will flow more easily. Inversely, the higher the grade, the thicker the oil, making its flow slower, which allows the formation of a protective film on engine parts.

As motor oil is affected by temperature, a grade gives information on the use and flow of the fluid when hot and when cold. An example:

  • At low temperatures, it is better to choose an oil with a low viscosity grade in order to facilitate circulation of the lubricant in your vehicle during a cold engine start;
  • Motor oil with a higher grade will be more resistant in the sensitive parts (hot areas) of the engine. As it is thicker, the oil will form a protective film on the engine parts, reducing wear and breakage by preventing friction between the parts.

Choosing the right viscosity grade, and therefore the right motor oil, is crucial to ensuring your vehicle runs smoothly. Oil helps cool your engine, lubricates the parts and, as we explained above, avoids friction and damage to the engine. In addition, motor oil facilitates the removal of impurities and efficiently inhibits the formation of corrosion and rust.

What are the different viscosity grades of a motor oil?

How do I know what the right viscosity grade is for my vehicle? The Society of Automotive Engineers (SAE) has developed a system for classifying oils according to their viscosity grade at low and high temperatures.

Monograde oils

Monograde oils are used over a relatively small temperature range. They are generally designed for older vehicles. This type of oil breaks down into two categories that depend on the season when you will be using your vehicle.

For winter use, choose an oil with the letter “W”, which stands for Winter. These are SAE 0W, 5W, 10W, 15W, 20W and 25W motor oils. Their viscosity grade is low, meaning they are particularly fluid lubricants. Each category is defined by its viscosity at a given temperature (from -10°C to -35°C depending on the grade). When cold, the more fluid the lubricant, the less work required by the oil pump at start-up.

To drive in the summer season and to take care of your engine, it is best to choose a motor oil with a high viscosity grade that is not marked “W”, meaning SAE 8, 12, 16, 20, 30, 40, 50 or 60 motor oils.

Multigrade oils

Multigrade oils are the most popular oils today because they are perfectly suited to current vehicle models. They also have the advantage of being usable in all seasons, regardless of the outside temperature. Multigrade oils are less affected by temperature variations than monograde oils. That’s why multigrade oil containers have a number on either side of the “W”. For example, on the most frequently purchased motor oils, you’ll see values such as 5W30, 15W40, or 10W40.

What do these viscosity grades mean? As with monograde oils, the “W” still means “Winter”. The number before the W represents the winter viscosity grade. In other words, the engine’s capacity to start, even at low temperatures. The smaller the number, the easier it is for the engine to start cold. As we have seen, for a fast start-up, you should choose a fluid oil.

The value after the W represents the motor oil viscosity grade at high temperatures. A high grade means optimized protection and sealing for your engine and its components, because a thick layer of oil is formed in the engine’s critical areas (hot areas). However, a less high grade will be more effective in reducing friction between hot parts and allows fuel savings.

If you have the slightest doubt when choosing the viscosity grade of your motor oil, check the automotive manufacturer’s recommendations, your vehicle owner’s handbook, or get the advice of a professional.