Difference Between 5W40 And 15W40

When it comes to choosing the right motor oil for your vehicle, it can be overwhelming. With so many different types and varieties of motor oil out there, it’s essential to understand the differences between them. Two of the most common types of motor oil are 5W40 and 15W40, and they both play an important role in keeping your engine running smoothly.

First, let’s break down what these numbers mean. The first number, in this case, either 5 or 15, represents the viscosity of the oil in cold temperatures. The lower the number, the better the oil will flow in cold temperatures. The second number, either 40, 50, or 60, represents the viscosity of the oil in high temperatures. The higher the number, the thicker the oil will be in high temperatures.

Now, let’s dive into the differences between 5W40 and 15W40.

Viscosity in Cold Temperatures

As mentioned before, the first number in the type of oil represents its viscosity in cold temperatures. This is important because when an engine sits for an extended period, the oil drains out of the engine and back into the oil pan, leaving the engine parts oil-free. When the engine is cold, thicker oil takes longer to reach these parts, potentially leading to inadequate lubrication and increased engine wear.

5W40 motor oil has a lower viscosity rating in cold temperatures, allowing it to flow easier and lubricate engine parts quicker. This helps to reduce wear and tear on the engine during cold starts. On the other hand, 15W40 motor oil is thicker in cold temperatures, which can take longer to reach engine parts and providing less initial lubrication.

Viscosity in High Temperatures

The second number in the type of oil represents viscosity at high temperatures. When the engine is running, temperatures inside the engine crankcase can exceed 200 degrees Fahrenheit. Under these high-temperature conditions, thicker oil performs better by maintaining its film strength, which protects the engine’s critical components from wear and tear.

15W40 motor oil is thicker at high temperatures than 5W40 motor oil, meaning it has a higher viscosity rating in high temperatures. This helps to maintain its film strength, provide better wear protection and prevent thermal breakdown.

Fuel Economy

5W40 oil is considered more fuel-efficient than 15W40 oil. Since 5W40 oil is thinner, it does not require as much power to circulate throughout the engine, and therefore, slightly more fuel-efficient.

Use For Diesel Engines

Both 5W40 and 15W40 oils are commonly used in diesel engines. However, 15W40 motor oil is more popular for diesel engines because it provides better high-temperature viscosity protection, which is essential for diesel engines, which can run hotter than gas engines.

Environmental Conditions

The environment plays a critical role in determining the right type of oil for your car. In regions with cold winters, 5W40 oil is highly recommended because it flows more easily at lower temperatures. In areas with hot summers and hotter temperatures, 15W40 oil is more advantageous since it offers better high-temperature performance.

Therefore, the type of oil you choose should be dependent on the environmental conditions in your area.


In summary, both 5W40 and 15W40 oils have their advantages and disadvantages, and choosing the right oil for your car should be dependent on the engine’s specific requirements, environmental conditions, and driving habits.

It’s always a good idea to consult the owner’s manual or reach out to a professional technician to determine which type of oil would work best for your vehicle. By following the recommended oil specifications, you can be assured of optimal performance, increased fuel efficiency, and a longer-lasting engine.