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What does 10W40 mean? - Explained by Cubic Cms

What does 10W40 mean? - Explained by Cubic Cms

Do you notice the word SAE 10W40 or SAE 5W40 on the box of a motorcycle or petrol car's engine oil or in the user manual book of a vehicle? Every notation contains a different meaning and uses differently as per the engine performance requirement with various hot and cold weather places. An appropriate oil grade enhances the performance and durability of the engine while an inappropriate engine oil causes low engine performance, engine oil leakage, wear and tear. If you don't know about it. We shall explain all about these oil grades in detail. Please read the full blog.

What does 10W40 mean - Image of Engine oil
Image by Vachagan Malkhasyan from Pixabay
These oils are called multigrade engine oil. In ancient times single grade oil was used in the engine which was sufficient for the only hot or cold conditions of the engine. As the engine oil heats up, its lubrication ability diminishes. Due to the need to deal with the cold and hot conditions of the engine, multigrade oil was discovered. The mineral base oil (for example SN100, SN150, SN500, or bright stock) is used as the main ingredient in this oil with some additives for high performance like the anti-foam agent, viscosity improver, etc. 

As we know the engine oil protects the engine from mechanical wear through high performance, high viscosity index, high dirt dispensing capacity, better cleaning capacity, and high thermal stability.

You should know the word viscosity first to understand these notations. In simple terms, viscosity defines how easily an engine oil flows, and the viscosity of an oil is measured by its resistance to flow. The viscosity of the engine changes with temperature change. As the engine is running, engine oil heats up, becomes less viscous, and flows easily.


  • Stands for Society of Automotive Engineers


  • Describe the viscosity or thickness of the engine oil at its coldest operating temperatures. It is tested generally at 0°C. The first number before W describes how an engine oil flows in the winter (cold weather) during engine ignition time from cold conditions (engine starting from off position).  


  • This shows the oil viscosity or thickness at higher operating temperatures. The oil viscosity or weight for hot temperatures is tested at 100°C. The second number after the word W describes how an engine oil flows when an engine is in running condition. 

We can also say that 10 and 40 represent the "performance or viscosity grade" at 0°C and 100°C respectively and W represents winter. In Fahrenheit, the range is from 0°F to 200°F and in Celsius, the range is from 0°C to 100°C. Both are considered approximately the same.  

The thinner is the engine oil the viscosity is low. In an engine, thin oil needs until it starts. Thin oil flows easily to protect the engine at cold temperatures. 

Thick oil means the oil with high viscosity. After the engine start, when the engine heats up, we need a thick oil that works to protect the engine at high temperature by maintaining a film strength of the oil.

Engine oil starts thickening when it cools and becomes thin after engine start-up as the engine temperature starts increasing.

The number before W supports the engine in cold (engine off to engine start-up) and the number after W supports the engine during engine normal operation conditions. We can say also that the first number shows viscosity at cold and the second number shows the viscosity at a temperature over 100°C.

Some assume W as a weight by mistake. Always remember that W represents the winter here. Sometimes 10W40 is written as 10W-40 or 10w40 though all are the same.    

We can understand this more clearly through examples. For example, 0W30 will flow easily at the time of engine ignition on than 5W30 because 0W30 is thinner than 5W30. Another example, a 10W30 will flow easily than a 10W40 at normal engine running conditions. It shows that with a smaller number, the oil will flow easily. A 30 viscosity grade oil is thinner than a 40 viscosity grade oil at 100°C.

How to choose the right oil for your vehicle:

This is the main concern. An inappropriate oil grade can harm your engine and gearbox in the form of leakage or inferior performance. You should consider these points to choose the appropriate oil for your vehicle-

  1. The viscosity
  2. The manufacturers recommended oil grades like 10W30, 10W40, etc.
  3. It is motor oil or gear oil
  4. Both the API and ACEA ratings    
  5. The OEM approvals

You can see these parameters in your vehicle's user book manual or service book manual.


  • API stands for American Petroleum Institute. API shows the vehicle's performance level categories. You should ask for API-licenced oil during the oil change of your vehicle. You can look at the API certification mark on the oil box in API No. where S represents the Petrol engine and C represents the Diesel engine.


  • ACEA stands for Association des Constructeurs EuropĂ©ens d'Automobiles - European Automobile Manufacturers Association. ACEA is equivalent to API. In ACEA numbers, A stands for the Petrol engine, B stands for Diesel engine and C stands for Catalyst compatible or low SAPS (Sulphate Ash, Phosphorus and Sulphur).

Assume you are using 10W30 oil and frequently facing a problem of engine leakage, tear and wear. Then you should use 10W40 oil to overcome these problems and more protection to the engine as it is thicker than 10W30. The oil which can retain its viscosity longer, the more efficiently it will lubricate an engine and prevent it from damages. 

10W30 is enough and suggested for cold weather places while 10W40 is best fit for the hotter weather places because in hot weather places 10W30 oil will get thin quickly which will degrade the efficiency of the engine oil to protect the engine so 10W40 is best suited and recommended for hot weather reasons.

SAE 5W30: 

  • For Europe (for cold weather places)

SAE 10W30 and 10W40: 

  • For India (for hot weather places)

Lastly, I shall say that don't go for low-cost oil because a low-cost product gives a low result. Saving a little amount in the engine oil purchasing can cost a much higher amount during the engine malfunction. So, always buy a certified good engine oil. Never change only engine oil, always do proper servicing of your vehicle at certified service centres. 

There are so many factors that keep the engine in condition and provide high performance with durability like service on time, air filter cleaning or changing, oil filter cleaning or changing, spark plug or injector cleaning and proper washing. A wisely chosen engine oil will keep the engine safe, durable, strong and safe. The requirement of engine oil grade can vary as per the use and weather conditions. Always remember that API and ACEA or at least API or ACEA approved engine oil has to be used in your vehicle.    

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