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10w30 vs. 15w40 — What's the Difference?

By Urooj Arif & Fiza Rafique — Published on February 19, 2024
10w30 is a multi-grade oil with a viscosity of 10 in cold temperatures and 30 at engine operating temperature. 15w40 is thicker, with a viscosity of 15 in the cold and 40 at operating temperature.
10w30 vs. 15w40 — What's the Difference?

Difference Between 10w30 and 15w40


Key Differences

10w30 and 15w40 are both multi-grade engine oils, but they differ in their viscosity ratings, indicating their flow resistance at different temperatures. The "10w" in 10w30 refers to the oil's viscosity in cold temperatures, making it thinner and easier to pump at lower temperatures. The "30" indicates its viscosity at the engine's operating temperature, suggesting it's relatively thinner compared to 15w40. This makes 10w30 suitable for lighter-duty engines and in regions with a wider temperature range, especially where lower starting temperatures are common.
Conversely, 15w40 oil is thicker at both low and high temperatures, as indicated by its higher viscosity ratings. The "15w" suggests that it's thicker during cold starts than 10w30, and the "40" denotes that it remains thicker under the engine's operating heat. This characteristic makes 15w40 more suitable for heavy-duty vehicles, diesel engines, and environments with higher ambient temperatures, as it provides better protection against wear under high stress and heat conditions.
Choosing between 10w30 and 15w40 often depends on the vehicle's engine specifications, the environmental conditions, and the type of use the vehicle sees. For example, 10w30 might be preferred for passenger cars and light-duty engines that operate in a broad range of temperatures, providing efficient lubrication and fuel economy. On the other hand, 15w40 is often recommended for trucks, buses, and construction vehicles, especially those operating in hot climates, as it maintains its protective properties at higher temperatures.
Both 10w30 and 15w40 oils contain additives that help protect engines from wear, deposits, and corrosion, but their different viscosities mean they perform optimally under different conditions. It's essential to consult the vehicle's owner manual or a professional mechanic to choose the right oil, as using the wrong viscosity can lead to increased engine wear, reduced fuel efficiency, and even engine failure.
Both 10w30 and 15w40 serve the primary function of lubricating and protecting the engine, their different viscosities make them better suited to different engines and operating conditions. Understanding these differences is crucial for maintaining engine health and ensuring optimal performance across the vehicle's lifespan.

Comparison Chart

Cold Temperature Viscosity

Thinner (10w) for easier cold starts
Thicker (15w) provides better protection in mild to hot climates

Operating Temperature Viscosity

Thinner (30) for light-duty engines
Thicker (40) for heavy-duty applications

Best Used For

Light-duty engines, broader temperature ranges
Heavy-duty engines, high temperature environments

Fuel Efficiency

Generally offers better fuel efficiency
Slightly lower fuel efficiency due to thicker consistency

Protection Level

Adequate protection for standard driving conditions
Enhanced protection under high stress and heat

Compare with Definitions


Provides good cold-start performance.
With 10w30, my car starts easily even on cold mornings.


Ideal for agricultural and construction machinery.
All our construction equipment is maintained with 15w40 for reliable performance.


Ideal for light-duty engines.
For my everyday vehicle, 10w30 offers the perfect balance of protection and efficiency.


Offers superior protection in hot climates.
In the hot summer months, I switch to 15w40 to keep my engine protected.


Enhances fuel economy in compatible engines.
Switching to 10w30 improved my car's fuel efficiency.


Preferred for heavy-duty engines.
Our fleet trucks run better on 15w40 due to its high-temperature stability.


Recommended for general passenger vehicle use.
My mechanic suggested 10w30 oil for optimal performance in my sedan.


Suitable for diesel engines requiring thicker oil.
For my diesel engine, only 15w40 provides the durability I need.


A multi-grade oil suitable for a wide range of temperatures.
I use 10w30 oil for my car because it provides smooth engine operation throughout the seasons.


Maintains viscosity better under high stress.
Under heavy loads, 15w40 oil keeps the engine running smoothly.

Common Curiosities

Is 15w40 better for hot weather?

Yes, 15w40 is generally better for hot weather due to its thicker consistency at operating temperatures.

What does 10w30 mean?

10w30 refers to the oil's viscosity rating, indicating it has a winter viscosity of 10 and an operating temperature viscosity of 30.

What does 15w40 mean?

15w40 indicates the oil's viscosity, with a winter viscosity of 15 and an operating temperature viscosity of 40.

Can I use 10w30 in a diesel engine?

While some diesel engines can use 10w30, it's essential to check the manufacturer's recommendation, as 15w40 is more commonly used.

Which oil provides better fuel efficiency?

10w30 typically offers better fuel efficiency due to its thinner consistency reducing internal engine drag.

Can I use 10w30 in winter?

Yes, 10w30 is suitable for use in winter due to its lower cold temperature viscosity.

Is 10w30 good for high-mileage engines?

10w30 can be used in high-mileage engines, but specific high-mileage formulas might provide better protection.

Can I mix 10w30 and 15w40 oils?

Mixing different viscosities is not recommended as it can alter the intended lubrication properties and affect engine performance.

How do I know if I should use 10w30 or 15w40?

The best way to determine this is by consulting your vehicle's owner manual or a professional mechanic for the manufacturer's recommended oil viscosity.

Does 15w40 oil last longer than 10w30?

Oil longevity depends more on the oil quality and operating conditions than the viscosity rating alone.

Is 15w40 only for trucks and heavy machinery?

While 15w40 is commonly used in heavy-duty applications, it can also be used in some passenger vehicles, depending on the manufacturer's specifications.

Can the use of 15w40 improve engine noise?

The thicker consistency of 15w40 might help dampen engine noise in some cases.

How often should I change 10w30 or 15w40 oil?

Oil change intervals depend on the vehicle's make, model, usage conditions, and whether you're using conventional or synthetic oil. Always follow the manufacturer's recommendations.

Does 15w40 protect better against engine wear?

Due to its thicker consistency at operating temperatures, 15w40 can provide better protection under high-load conditions.

Are there synthetic versions of 10w30 and 15w40?

Yes, both 10w30 and 15w40 are available in synthetic formulations, offering improved performance and protection.

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Author Spotlight

Written by
Urooj Arif
Urooj is a skilled content writer at Ask Difference, known for her exceptional ability to simplify complex topics into engaging and informative content. With a passion for research and a flair for clear, concise writing, she consistently delivers articles that resonate with our diverse audience.
Co-written by
Fiza Rafique
Fiza Rafique is a skilled content writer at, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.

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