5W 40 Vs 15W 40

When it comes to choosing the perfect oil for your vehicle, it can be overwhelming to understand which one to use. Two of the most commonly used oils in the automotive industry are 5W-40 and 15W-40. Both are multi-grade oils that are used for diesel engines and provide several advantages. However, when comparing 5W-40 vs. 15W-40, it is essential to understand the differences and how they can affect your vehicle’s performance.

5W-40 Oil

5W-40 oil is a fully synthetic oil that is designed to provide high performance and protection for diesel engines. The “5W” in the name indicates that this oil has a low winter viscosity rating, which means it flows well in cold temperatures. This oil is perfect for diesel engines that are used in colder northern climates, where the temperatures can get below freezing.

Benefits of using 5W-40 oil:

1. Improved Fuel Efficiency

Using 5W-40 oil can help improve fuel efficiency in diesel engines. Due to the low viscosity rating, the engine can have less resistance, which results in less fuel consumption and reduced emissions.

2. Enhanced Engine Protection

This oil provides high levels of protection against wear, corrosion, and oxidation. It also has a high detergency that helps keep the engine clean.

3. Cold Start Protection

5W-40 oil provides excellent cold start protection. Due to lower viscosity, it can quickly circulate through the engine and lubricate the parts, which reduces wear and tear.

4. Extended Drain Intervals

With advancements in synthetic oil technology, 5W-40 oil can last for an extended period than conventional oil. This oil also has fewer impurities, which means it can go longer between oil changes.

15W-40 Oil

15W-40 oil is a mineral-based oil that is known for its durability and versatility. The “15W” indicates that it has a higher winter viscosity rating, which means it is better suited for warmer climates. This oil is commonly found in diesel engines used in trucks, tractors, and boats.

Benefits of using 15W-40 oil:

1. Improved Low-Speed Torque

15W-40 oil has a higher viscosity, which means it can provide better low-speed torque in diesel engines. This oil can also help with high load applications.

2. Better Abrasive Wear Protection

Due to the high-viscosity rating, 15W-40 oil can provide improved abrasion wear protection. This oil can also withstand high temperatures and pressure, which can extend the engine life.

3. Reduced Engine Deposits

15W-40 oil has detergents that prevent the formation of engine deposits such as sludge and varnish. This oil can also help keep the engine clean and improve performance.

4. Better High-Temperature Stability

This oil can maintain its viscosity at high temperatures, which is important for diesel engines that run hot. This oil is less prone to breaking down when exposed to extreme temperatures.

5W-40 vs. 15W-40

When comparing 5W-40 vs. 15W-40 oil, there are some significant differences to consider. While both oils have advantages and disadvantages, each oil is designed to work in specific conditions.

1. Viscosity

The primary difference between these oils is viscosity. 5W-40 oil has a lower winter viscosity rating, which means it flows better in colder temperatures. 15W-40 oil has a higher viscosity rating, which means it is better suited for warmer temperatures and can provide better low-speed torque.

2. Fuel Efficiency

5W-40 oil can provide better fuel efficiency due to the lower viscosity rating, which reduces engine resistance. However, this advantage may not be significant in warm climates where 15W-40 oil is better suited.

3. Protection

Both oils provide high levels of engine protection, but 5W-40 oil offers higher detergency, which can keep the engine cleaner. 15W-40 offers better abrasive wear protection due to the high-viscosity rating.

4. Durability

Both oils are durable and can withstand high temperatures and pressure, but 5W-40 oil lasts longer between oil changes due to fewer impurities.


1. Can I use 5W-40 oil in a gasoline engine?

Yes, 5W-40 oil can be used in gasoline engines that require the same viscosity rating. However, it may not provide the same advantages in gasoline engines that it does in diesel engines.

2. Can I use 15W-40 oil in a cold climate?

15W-40 oil can be used in colder climates, but it may not flow as well as 5W-40 oil in extreme cold temperatures.

3. Should I use synthetic or mineral-based oil?

It depends on your vehicle and driving conditions. Synthetic oil provides better performance and protection but is more expensive. Mineral-based oil is more affordable but may not provide the same level of protection.


Choosing the right oil for your diesel engine is essential for maintaining its performance and longevity. When comparing 5W-40 vs. 15W-40, consider the viscosity rating, fuel efficiency, engine protection, and durability. Ultimately, the best oil for your vehicle depends on your driving conditions and what your vehicle’s manual recommends.