Understanding Oil Weight: What Do Oil Numbers Mean?

Oil weight is a term that often confuses car owners and automobile enthusiasts. What is oil weight meaning and what do oil numbers mean? You need to know these things to make sure that the car engine has a healthy life. Otherwise, you cannot choose the right type of oil as recommended by the car manufacturer.

Have a look at the classification of oil and the meaning of oil numbers in the following sections.

What Is the Meaning of “Weight” In This Context?

In the term “oil weight”, “weight” does not mean mass. Instead, it means the viscosity of the oil. In simple terms, the weight of the oil means how smoothly it can flow at a specific temperature.

The standard operating temperature for most car motors is 210 degrees F. The Society of Automotive Engineers (SAE) allocates a viscosity number (weight in another term) to each type of oil depending on its flowing capacity at that temperature.

What Do the “Numbers” on Oil Mean?

The vehicle engine will face various internal issues if you do not use the right engine oil. Hence, having a complete idea about the kind of oil is compulsory. In every weight number, there is a “W”. Now the question is, what does “W” mean here? In this case, “W” stands for winter and denotes the oil viscosity in cold temperatures.

Since the temperature has a substantial effect on the oil viscosity, there is a need for oil of different grades for usage in different weather. For instance, it could be perfect to use oil with 40 W for a long road trip in summer when the engine is already hot. But it will be a disaster to use the same oil in winter. Contrarily, 5 W is the right choice for wintertime with single-digit temperatures. But once the snow melts, it will fail to provide sufficient lubrication.

oil weight meaning
Oil weight indicates its viscosity. (Source: Dvortygirl / Wikimedia Commons)

The higher weight number means the oil is thicker and will flow slowly. It signifies the nature of oil while coating internal engine components and protecting them against heat and friction. For example, there are two types of oil with a weight of 30 and 50. Between these, 50 weight oil will flow slower than the 30 weight oil, but the first one will offer an extra level of protection in stressful conditions or high temperatures. However, lower oil weight means thinner or the most viscous oil.


Things to Consider about Oil Weight

Using high viscosity oil can result in excessive temperature in oil and increased drag. On the other hand, low viscosity oil will cause high friction between the moving parts of the car.

Using a correct viscosity oil will enable easy starting of the vehicle, besides reduced friction and slow wear. Moreover, you can enjoy improved fuel economy and more horsepower by using the right kind of oil.


Oil weight bears high importance when it comes to the life of your engine. The type of oil you choose should coincide with the driving weather and the manufacturer’s recommendation. The discussion on oil weight explained here will give you a better understanding.