What Does 15W40 Mean


You must have come across a three-digit alphanumeric code on most of the engine oil. But what does it really stand for? You’re asking for a perfectly good reason. 

You’ve read online that the numbers are telling you how your engine oil viscosity should be. But still, is it “weight w 40” or “winter w 40”? Or, perhaps, something completely different? In this blog, we want to explain to you what the numbers in viscosity grades actually mean. Let’s find out.

To simply put it, 15w40 is a number code to specify the range of viscosities of engine oils at specific test temperatures established by the Society of Automotive Engineers (SAE). To understand what the numbers and the letters stand for we first must understand what engine oils are.

Types of engine oil

Engine oil is one of the most essential elements when it comes to a smooth-running engine. The engine oil not only reduces friction by lubricating the internal parts of the engine but also maintains an optimum core internal temperature. Some of the other key functions of the engine oil are to neutralise the acids from the combustion process, clean out residue from the combustion chamber and prevent corrosion and oxidation. Understanding the various types and grades of engine oils is pivotal and will help us make a better choice the next time we’re at a service centre.


There are essentially three types of engine oil. Mineral oil, synthetic oil, and a blend of the two called – semi-synthetic oil.

  1. Mineral oil – This is essentially refined petroleum oils that undergo treatment for a wider range of uses and temperatures as they offer very little to no protection or lubrication against friction-induced heat. They perform inefficiently at low temperatures and need to be changed more often as well.
  1. Synthetic oil – Fully synthetic oils are the best oils available. They go through extensive research and treatment in laboratories. They offer excellent lubrication and run well at varied temperatures. They provide excellent engine protection and improve fuel efficiency.
  1. Semi-synthetic oil – It sits in between the two oil types offering the affordability of mineral oil and some of the performance of the synthetic oil. A small amount of synthetic oil is blended with mineral oil to make it slightly superior. The addition of synthetic oil improves viscosity and resistance to heat at higher temperatures. They are better than mineral oils at lower temperatures.


The SAE (Society of Automotive Engineers) has specified different grades of oils based on their viscosity. Viscosity is the ability to resist flow, in other words, it defines the thickness of the oil. Various oils have different viscosities at room temperature and oils also behave differently at different temperatures.

What does 15W40 mean?

The oil grade is in the form of an alphanumeric code. Each alphabet and number denote something very specific. Let’s take a look and understand what it means.

  • A set of numbers is followed by a “W”.
  • W” stands for winter
  • The numbers before the (W) “15” denote the temperature a cold start can be performed at.
  • The numbers after (W) “40” denote how finely the oil will flow at normal running temperatures.

The higher the number, the thicker the oil. The lower the number, the thinner. Thinner oils flow faster than thicker oils, and manufacturers design engines to work with a certain grade/ or a range of grades of oil. This means a 15W40 will have a better flow than 20W40 at starting and normal running temperatures.

Engine oil grade

Most manufacturers will suggest using one particular grade of oil while others might suggest a range of grades based on the climate and temperatures in which the vehicle is driven. Changing your engine oil from time to time and using the correct grade oil can increase the life of the engine and its internal parts significantly.  If at any time you are unsure of what engine oil works for your vehicle, or what is best for your climatic conditions, it is always safer to follow the manufacturers’ recommendations. 

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