Oil Viscosity Numbers Really Do Matter! Understand SAE Ratings (10W40 etc)!
The SAE viscosity numbers on engine oil are easy to grasp and when you know the basics you'll be better prepared to choose the right oil for your car and your region.
Understanding multi-grade oils and their viscosity ratings can be confusing, but it doesn’t have to be. In this video, we break down what those numbers and letters on your oil container really mean.
Ever wondered what 0W40 actually signifies? We’ll cover the basics of oil viscosity, what those ratings indicate, and how to choose the right oil for your car depending on your climate and driving conditions.
The Evolution of Engine Oils
Back in the day, drivers used different oils for summer and winter. Why? Because oils had limited operating ranges.
Thinner oils flowed better in the cold, while thicker oils were ideal for hot summers. Today’s multi-grade oils, like 0W40, are designed to perform well across a range of temperatures, making them more versatile and convenient.
What Does 0W40 Mean?
The first number, before the "W", is the winter rating, indicating how well the oil flows at low temperatures.
For example, 0W oil flows well even at -40°C, making it ideal for extremely cold climates. The second number, like the 40 in 0W40, measures the oil’s viscosity at 100°C. It’s consistent across all oils with the same rating, meaning a 0W40 oil and a 15W40 oil will have the same viscosity at high temperatures.
Why Viscosity Matters
Viscosity refers to the oil’s resistance to flow—think of it as how “sticky” the oil is. A higher number means a thicker, more viscous oil.
But don’t be tempted to just pick the thinnest or thickest oil you can find. The engine’s internal clearances change as it heats up, so the right oil ensures proper lubrication, even in those tight spots.
Choosing the Right Oil
It’s not just about temperature. Engine design, wear and tear, and even your driving habits should influence your choice of oil.
Higher-mileage engines might benefit from a different grade, but for most drivers, sticking to the manufacturer’s recommendation is the best bet. Regular oil changes are also crucial for maintaining engine reliability.
The Science Behind It
The viscosity of fluids is measured in centiStokes (CST). For example, water at 20°C has a CST just over 1, while honey can have a CST as high as 7,000.
But you won’t see CST on oil bottles because it would make things even more confusing. The SAE ratings you see—like 0W40—offer a simpler way to choose the right oil for your vehicle.
More Oil Insights
Oil viscosity is just one piece of the puzzle. We’ve got more videos that dive into other important factors like oil formulations, additives, and the consequences of overfilling or underfilling your engine.
Check out our playlist for more tips and advice. If you found this video helpful, hit that like button and subscribe to our channel for more car maintenance insights. Thanks for watching, and see you in the next video!
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Legal Notice: Unless we have inspected your car we can only provide generic theory. All information is provided without warranty, please check any recommendations made with a mechanic locally to verify it would be legal in your area or region and that it would be suitable for your car and your needs.