10W30 vs. 10W40 – The Ultimate Differences Explained

When the car engine runs, oil pumps through it and protects from mechanical wear. Nowadays, the automobile industry is flooded with a variety of types, weights, and formulation of motor oil. 10W30 and 10W40 are the popular viscosities used in modern engines. The differences between 10W30 vs. 10W40 viscosities aren’t that big. Gaining an understanding of the working of the grading system is necessary to know about the differences along with fluid viscosity and behavior of oils.

Engine Oil Viscosity – Differences between 10W30 Vs. 10W40

As the technological advancements are happening at breakneck speed, motor oil has also covered a long way from what it was in its early beginning. Since oil is more resistant to flow, water flows at a faster rate. Therefore, Viscosity is an essential element of a lubricant.

Car engine oil lubricates various parts of the engine. One of the best maintenance tips is to buy engine oil that remains unaffected with the fluctuations in the temperature. This blog will be of great help for those who want to know about the differences between 10W30 and 10W40. Let’s begin with known viscosity of the fluid and oil behavior under various temperatures.

Oil Weight

The thickness or measurement of the oil viscosity becomes easy when reading the numbers like 10W30 will help know oil weight. The oil is thinner in case of lower weights, while higher weights show thicker oil.

In a car engine, knowing the movement of the oil will help understand its uses. For example, thinner oil is best during the initial stages. However, it can be a cause of concern when the engine heats up.

10W30 vs. 10W40
Car engine oil lubricates various parts of the engine. (Photo Source: pixabay)



The viscosity of a liquid continually changes because the oil runs through the engine. As discussed, motor oil adjusts with the changes in temperature. Due to the increased motion of the molecules, the oil becomes less viscous when it heats up. Therefore, it is imperative to keep it in mind when purchasing categorizing oil.

Multi-Weight Oils

In order to keep the car engine functioning well, it is necessary to know about the change in viscosity. The thin oil is perfect when the engine is cold. However, the same is not ideal when the engine is hot. Therefore, finding multi-weight oil is the surefire way to be assured that oil will not thin out too much when the engine works. 10W30 vs. 10W40 differences aren’t much because of the same weight when cold. As temperature changes, the long-chain polymers present in oils expand and contract accordingly. They alter the way the oil behaves. Since they are designed as per the needs, they are thin when cold, and not thin at high temperatures.

10W30 vs. 10W40
10W40 engine oil retains the viscosity longer than 10W30. (Photo Source: shutterstock)


The number before the W tells that the oil weighs in cold condition. During the weight measurement of multi-weight oil, having an understanding of the numbers will help immensely. The second number shows the temperature of more than 100 degrees centigrade. Multi-weight oils are a preferred choice in the automobile industry because they are thinner when hot as compared to when cold. 10W30 oil changes accordingly with temperature, such as it is then when hot as 30- weight oil, and thinner as 10-weight oil when cold.

Decoding the Figures  

Both 10W30 and 10W40 oils move through the engine quickly. The letter ‘W’ refers to winter, which separates these two sets of numbers. The fluid’s tendency to restrict flow or thickness of engine oil is related to viscosity.

The number before the letter ‘W’ denotes the viscosity in cold temperature, usually of zero degrees. The oil becomes viscous when the temperature is cold. So, it means that the engine oil rated OW20 will circulate more quickly throughout the engine as compared to oil with 10W ratings. Though the 10W rating engine will start the car, OW will not take that much time to warm up the engine.

Now, the second number shows the viscosity of the engine while running at up to 212 degrees. The oil should be thick because it protects the heat-sensitive components of the vehicle engine. Put simply, if the value of viscosity is higher, it is thicker in hot temperatures.

Now, that all numbers are decoded, differentiating 10W30 vs. 10W40 engine oil becomes easier. Both oils have the same cold temperature viscosity rating – ‘10W’. Therefore, they resist becoming solid at zero degree temperature.

When noticing the second part of the rating – 30 vs. 40 – it is evident that 10W40 engine oil is more likely to retain the viscosity longer than then engine oil with a rating of 10W30.

10W30 vs. 10W40
Long-chain polymers present in 10W30 and 10W40 oils expand according to temperature. (Photo Source: whichcar)


Thus, using the rating specified engine oil assures sufficient lubrication of internal components. The difference between 10W30 vs. 10W40 viscosities is the thickness at hot temperatures. During the cold temperature, they flow at the same rate.