Engine oil viscosity grades explained

5W30, 15W40, 10W40 – we’ve all encountered designations like these when choosing engine lubricants, but what do they mean and why do they matter when choosing engine oil? These figures represent the engine oil’s viscosity grade, which is the lubricant’s level of fluidity and efficiency at low and high temperatures. In this guide, we look at the various types of vehicle lube grades and what they mean – here are the viscosity grades explained.

What is the viscosity grade of engine oil? 

The viscosity grade of engine oil provides information on the oil’s resistance to flow inside the 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, slowing its flow to help form a protective film across engine parts.
As engine oil is affected by temperature, its grade gives information on the use and flow of the lubricant when hot and cold. For example:

• At low temperatures, it is better to choose an oil with a low viscosity grade to facilitate circulation of the lubricant in your vehicle during a cold engine start.
• Engine oil with a higher grade will be more resistant in the hotter, more sensitive parts 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 parts.

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

 What are the different viscosity grades of engine oil? 

How do you know what the right viscosity grade is for your 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 and are generally designed for older vehicles. This type of oil breaks down into two categories, depending on the time of year when the vehicle is in use.
For colder, wintry months, choose an oil marked with the letter ‘W’, for ‘winter’. These are SAE 0W, 5W, 10W, 15W, 20W, and 25W engine 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 is required by the oil pump during start-up. 
To take care of your engine drive in the warmer months, it is best to choose a engine oil with a high viscosity grade that is not marked ‘W’, meaning SAE 8, 12, 16, 20, 30, 40, 50, or 60 engine oils. A higher viscosity grade (thicker oil) will ensure that the engine is properly protected and sealed in warm weather, although this can come at the loss of the better lubrication and fuel-saving capabilities of lower-grade oils.

Multigrade oils 

Multigrade oils are perfectly suited to contemporary vehicle models, making them the most popular oils today. They have the advantage of working effectively in all seasons, regardless of the outside temperature, because they are less affected by temperature variations than monograde oils.
Multigrade lubricant containers feature a number on either side of the ‘W’ to denote the oil’s ability to cope with seasonal temperature variations, such as 5W30, 15W40, or 10W40.
What do these viscosity grades mean? As with monograde oils, the ‘W’ still means ‘winter’. The number before this ‘W’ represents the winter viscosity grade – the engine’s capacity to start, even at low temperatures. The lower the number, the easier it is for the engine to start cold, so for a faster start-up, you should choose a more fluid oil.
The value after the W represents the engine oil viscosity grade at high temperatures. A higher-grade means optimized component protection and engine sealing, because a thick layer of oil is formed in the engine’s hotter, more critical areas. Conversely, a lower number will be more effective in reducing friction between hot parts and improving fuel consumption.
 
If you have any doubt when choosing the viscosity grade of your engine oil, check your vehicle owner’s handbook, get the advice of a professional or visit Total’s oil search and reference tool, LubConsult. 
Find out more about engine oil or view TotalEnergies’ range of automotive lubricants.