Understanding the Viscosity Grade of an Engine Oil

Published Date - 8th Jan 2020

When purchasing an engine oil can, you may have come across values such as 10W30, 5W40, and 0W20 on the can. What are these values? These values determine the levels of viscosity of the oil. The viscosity grades are determined as per the oil performance in low and high temperatures. The question arises, how do we know which viscosity grade of an engine oil fits our vehicle? Read further to get better clarity.

Viscosity is determined by the fluid’s resistance to flow. The viscosity grade of an engine oil provides information on the oil’s resistance to flow in your vehicle’s engine. An engine oil with a low viscosity grade will be more fluid and will flow easily. On the other hand, the engine oil with a higher grade is thicker and makes the flow slower which allows the formation of a protective layer on engine parts.

Engine oils are affected by the temperature, a grade gives information on the use and flow of the fluid when hot and cold temperatures exist. At low temperatures, an engine oil with a low viscosity grade is advised as it facilitates circulation of the lubricant in your vehicle during a cold engine start. At high temperatures, engine oil with a high viscosity grade is preferred as it will be more resistant in the crucial areas of the engine (hot areas). As it is thicker, it reduces wear and breakage and prevents friction between parts.

Picking the right engine oil for your vehicle’s engine to run smoothly. Engine oil reduces friction between parts, cools your engine, releases impurities, and inhibits the formation of corrosion in the engine circuit.

The Society of Automotive Engineers (SAE) has developed a system for classifying oils according to their viscosity grade at low and high temperatures. Here are the different engine oil grades per viscosity:

Monograde Oils:

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

Monograde oils fall into two categories:

The low engine oil viscosity grades end with a “W” and are usually suited for winter use. For Eg: SAE 0W, 5W, 10W, 15W, 20W and 25W engine oils. The high engine oil viscosity grades are not marked “W” and are suited for summer use. For Eg: SAE 8, 12, 16, 20, 30, 40, 50, or 60 engine oils.

Multigrade oils

Multigrade oils have gained popularity because they suit current vehicle models well. They also have the advantage of being usable in all seasons.

Multigrade oils must fulfill two viscosity specifications, their viscosity grades consist of two numbers. For Eg: 10W30. 10W is the low-temperature viscosity (winter) whereas 30 is the high-temperature viscosity oil (summer). Multigrade oils are less affected by temperature variations than monograde oils. It is due to this that multigrade oil containers have a number on either side of the “W”. On the most frequently purchased engine oils, you will see values such as 5W40, 20W40, or 10W30.

When in doubt choosing the viscosity grade of your engine oil, check the owner’s manual, and engine oil specifications by the vehicle’s manufacturer or seek professional advice.