Engine oil grades usually define two things: the viscosity in low temperatures and high temperatures. Understanding the difference between engine oils with different viscosity levels is important for the well-being of your engine.
So, what are the differences between 15w40 and 20w50? 15w40 has a lower viscosity at low temperatures, so it is better for cold weather. 20w50 is more suitable for warm weather because it has a higher viscosity when hot.
These two engine oils have some more differences and a few similarities. I will explain all of their characteristics to help you choose the right oil. Let’s get started.
15w40 vs 20w50: Comparison Chart
The following chart contains the key differences and similarities between these two engine oils. Take a closer look.
| Properties | 15w40 | 20w50 |
| Viscosity Index | 143 | 141 |
| Working Range | -25°C to 40°C / -13°F to 104°F | -20°C to 40°C / -4°F to 104°F |
| Kinematic Viscosity | 14.9 sq.mm/s | 19.0 sq.mm/s |
| Low-Temperature Viscosity | Lower | Higher |
| High-Temperature Viscosity | Lower | Higher |
| Oil Consumption | Higher | Lower |
| Fuel Consumption | Lower | Higher |
| Flashpoint | 237°C / 458.6°F | 230°C / 446°F |
| Suitable for | Gasoline engines | Diesel engines |
| Price | $15.98 | $22.96 |
What Are 15w40 and 20w50?
These two are heavy-duty multi-grade engine oils. Multi-grade means these oils have two different viscosity levels at different temperatures.
For example, 15w40 has two viscosity levels. When the temperature is low, it will have a viscosity level of 15. It means the oil will be much thinner at low temperatures. As the oil gets hotter, the viscosity will start to change.
When the oil is hot, its viscosity level will be 40. So, the oil will become much thicker at operating temperatures.
The same explanation applies to 20w50, but the numbers are a bit different. You might already assume 20w50 will have a viscosity level of 20 at low temperatures. It means this oil will be a bit thicker than 15w40 at low temperatures.
At high temperatures, its viscosity level will increase up to 50. So, it will have a higher viscosity than 15w40 at operating temperatures. I will explain the performance of these oils in a bit.
15w40 vs 20w50: Similarities and Differences
You might have already checked the comparison chart. Let me explain how these properties impact the performance.
Viscosity Index
Many people will confuse it with the viscosity of the oil. But it is actually the rate of change of viscosity in regard to the temperature. 15w40 has a higher viscosity index, which means its viscosity will change more when the temperature is changed.
20w50 has a slightly lower viscosity index. As a result, its viscosity change will be a bit slower than 15w40.
Low-Temperature Viscosity
When the temperature is low, 15w40 will have a lower viscosity. For example, if you are driving in chilly weather, 15w40 will be thinner. As a result, it will flow quickly through the engine and lubricate engine components faster.
The lowest operating temperature of 15w40 is -25°C or -13°F. So, this oil can work in extremely cold weather conditions.
20w50 has a slightly higher viscosity than 15w40 at low temperatures. So, it will flow slower than 15w40. Its lowest working temperature is -20°C or -4°F. However, it is suitable for cold temperatures.
High-Temperature Viscosity
The number on the right indicates the high-temperature viscosity of these oils. 15w40 will be thicker at high temperatures. For example, when the oil temperature is 40°C or 104°F, it will have the highest viscosity. So, the oil will protect engine components from wear.
20w50 has an even higher viscosity than 15w40 at high temperatures. At 40°C or 104°F, it will have a viscosity level of 50. So, it will be much thicker than 15w40 at high temperatures.
Oil Consumption
You might already know thinner oils are consumed more by the engine. 15w40 is a thinner oil than 20w50. It flows faster through the engine when the engine is cold. As a result, the engine will consume 15w40 more than 20w50.
20w50 is a thicker oil than 15w40. Its flow is slower, so the engine will consume 20w50 less. As a result, the oil consumption is higher when you use 15w40.
Fuel Consumption
When the engine consumes oil faster, it can operate more efficiently. As a result, the engine requires less fuel. But as the oil gets thicker, lubricating the engine components takes more time.
So, the engine efficiency is lower in the beginning. It results in a higher fuel consumption.
Engine Protection
Both 15w40 and 20w50 are great in terms of engine protection. Due to superior base oils and special additives, these oils cause a very low sludge buildup.
These oils also break down very slowly, which causes the engine to perform better for a longer period. As both oils are thick at high temperatures, they create a protective film between engine components.
The thickness prevents engine wear and improves the service life of the engine.
Suitability
15w40 is commonly used for gasoline engines that have a lower load capacity. This oil also helps in cold starts, so it is suitable for areas with freezing temperatures.
20w50 is a more popular option for heavy-duty diesel engines that run for a long time. For example, if you have long-haul trucks, 20w50 is a better choice. These engines get much hotter at long stretches where the higher thickness of 20w50 works great.
Price
15w40 has a retail price of $15.98, and 20w50 is priced at $22.96. The price may vary slightly depending on the oil brand.
Which Is Better Between 15w40 and 20w50?
There is no discrete parameter to declare one of these oils better. It depends on your engine specifications, load capacity, driving conditions, running time, etc.
If you drive a light-duty truck or passenger car in cold weather, 15w40 will be better. But 20w50 is better if you drive in a hot, humid climate with greater loads. So, you should use your discretion and the suggestion of the manufacturer to choose the right oil.
FAQs
Check answers to some commonly asked questions about 15w40 vs 20w50.
Q: Are 15w40 and 20w50 interchangeable?
Even though these oils have differences, they can be used interchangeably in certain situations. For example, if you drive in a moderate temperature condition, they can be interchanged.
Q: Can I mix 15w40 with 20w50?
The viscosity indices of these oils are very close. So, you can mix them with each other. It won’t greatly impact the engine performance.
Q: Can I use 15w40 in a heavy diesel engine?
Heavy diesel engines need thicker engine oils to protect engine components. So, it is better not to use 15w40 in such an engine.
Each engine oil grade is suitable for specific engines at specific temperatures. To make this more understandable for general users, engine oils have different grades. You need to know what these grades mean to choose the right oil.
I compared 15w40 vs 20w50 to tell you the differences and similarities between them. You have also learned how they perform in different conditions.
So, it will now be much easier for you to choose the right engine oil. Remember, your choice of engine oil will impact the engine in the long term.