What Do Oil Numbers Mean?

Have you ever visited a motor oil store and noticed that each oil container has numbers and letters written on it? These numbers usually start with SAE before a number and a letter. For example, SAE 5W 40 and SAE 10W 40. What do these numbers and letters mean, anyway?

The numbers are used to represent oil weight in the industry. However, this is a bit misleading. In the automotive industry, oil weight does not mean how heavy the oil is, but it is used in measuring the oil thickness or viscosity. For example, water has a low viscosity, while Syrup has a high viscosity index. The lower the number on the bottle, the thinner the oil.

In this article, I’ll answer the question, what do the numbers on oil containers mean, and also explain oil viscosity. Grab a seat and a cup of coffee while I walk you through all you need to know.

engine oil grades chart

What do oil Numbers Mean?

As I pointed out earlier, the numbers and letters on the motor oil containers represent oil viscosity. Let’s have oil viscosity explained for better understanding. Oil viscosity is how easily motor oil flows in the engine at different temperatures.

For instance, thin oils have lower viscosity and flow more easily than thick oils with higher viscosities. The lower the numbers, the lower the viscosity; the higher the number, the higher the viscosity. Thin oils are better for cold temperatures so that they can flow easily.

The oil viscosity, the numbers you see on motor oil containers, ranges from 0-60 and is determined through several lab tests conducted by the Society of Automotive Engineers (SAE).

The lowest number represents the thinnest motor oil and the highest represents the thickest oil. For example, 0W-40 is more viscous than 5W-30. The W means ‘Winter’ and shows the oil’s viscosity in winter or cold climate areas.

Believe it or not, winter has a way of affecting oil viscosity. So, when to use thicker oil? It is best for the summer season. However, you must stick with the manufacturer’s recommendation and only make slight adjustments when necessary.

For instance, 40-weight oil will work fine on long-trip vehicles in the summer but will form engine sludge in winter. Meanwhile, 5-weight oil is best for cold winter seasons but will provide little lubrication during hot summer.

If you are not sure of the recommended oil viscosity for your vehicle, check your owner’s booklet. However, some situations may require you to change your oil grade.

Read Also: Can I use 10w30 Instead of 5w30?

Cold climate areas

You can switch to a thinner oil if you live in cold weather areas and do more stop-and-go driving. Lower oil viscosity works better in cold climate areas and prevents accelerated wear on the internal engine parts.

High mileage vehicles

Internal engine parts like the crankshaft, connecting rods, and camshafts wear off over time. As a result, there will be extra space between high-mileage engine components compared to brand-new vehicles of the same make and model. These high-mileage cars will need thicker oils to fill in the extra space.

With all these said, what does the second number in oil mean? I’ll explain that in the subsequent paragraphs.

what does the second number in oil mean

What does the second Number in oil mean

You don’t have to change engine oil when traveling to icy locations and back to regions with hot weather. Luckily, the SAE and oil producers have that in mind, and that’s why we have the first and second numbers on most oil containers. If you are wondering what do the two numbers on motor oil mean? It means most oils have multi-viscosity, meaning they can perform well in cold and hot weather conditions.

For example, a 5W-40 oil has a 5-weight rating when cold but works up to a 40-weight temperature when the engine is running. So if you were wondering what does 5w 40 oil means, you now know the answer.

Also, if you are asking, what do oil numbers mean 10w 40? It means the oil has a 10-weight at winter or cold start-up and also works better at a 40-weight temperature when the engine is running. From this, you can see that the second number in oil means the viscosity at normal engine operating temperature. 

Final Words

At this point, you’ll no longer ask what do oil numbers mean. They mean oil viscosity as tested by the Society of Automotive Engineers. The lower the number, the thinner the oil, and vice versa. Thinner motor oil produces less friction. This means they flow more easily than thicker ones.

Auto manufacturers are designing modern engines to run on lower numbers. No wonder oil numbers are getting lower these days. Some oil numbers are as low as 0w rating, which enhances engine performance in cold climates.

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Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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