10w30 vs 10w40: Engine Oil Viscosity Differences

Engine oil is a lubricant that coats all of the metal components of an engine. Using a motor oil that's recommended by the manufacturer helps to protect against friction and heat, extends the engine's life, and prevents premature wear and tear.

If you're unsure whether to opt for 10w30 or 10w40, we discuss the benefits of each of the oil grades and the advantages or disadvantages they offer to your driving experience and engine longevity.

When comparing oils there are multiple factors to consider. Regular driving conditions, external environment, climate, and the number of miles the vehicle has.

In most cases, 10w40 oil is almost always better unless you make many short trips in winter as 10w30 oil is less viscous when cold.

One factor that shouldn't be ignored is the manufacturer's recommendation. When in doubt, always defer back to the Owner's manual.

What the Numbers on Motor Oil Represent

mobil 1 motor oil

Before you can decide between types of oil, it helps to understand their classification and definition.

Viscosity Rating

A viscosity rating is achieved by testing oil at two industry set temperatures, rating a fluid's resistance to flow.

The first test occurs at 32°F and the second at 212°F. It is quantified thus:

(Number) W (Number)

To illustrate with the article's example: 10 W 30 or 10 W 40

Where the first number is the Cold Temperature Viscosity and the second the Hot Temperature Viscosity.

W stands for WINTER and not weight as some people might suggest.

Oil's viscosity is fully impacted by many factors, particularly temperature.

It is inversely proportionate; if the temperature is high the oil becomes less viscous. When faced with cold temperatures oil maintains a thicker consistency and becomes more viscous.

As an engine heats from cold to very hot, the viscosity of the motor oil thins and behaves differently. High viscosity oil is thick and runs slowly, low viscosity oil runs much faster.

Low viscosity oils offer greater engine protection in cold, wintry conditions. High-viscosity oils provide better engine protection in hotter climates.

Simply put, the first number denotes the thickness of the oil, its ability to maintain resistance to thickening, and how easily a car starts in cold weather.

The second number describes how thin the oil becomes when the engine is hot and how it behaves in hot weather, and its ability to maintain resistance to thinning.

Lower numbers = more viscous/thinner oil

Benefits of 10w30 Oil

10w30 motor oil

Improved Fuel Efficiency

Well-lubricated engine components work effortlessly and have less demand placed on them. Less fuel is consumed when there is less friction to work against. Over a year this can be as much as 1.6% improved fuel economy. If your annual fuel usage is high, this is a noticeable saving.

Year-Round Use

It is durable and consistent in all operating conditions and at multiple temperatures. The oil remains flexible enough to provide a protective coating in both hot and cold climates.

Cold Starts

Ideal for those early, cold mornings when the thinner oil is rapidly pumped over the cold engine providing an immediate protective coating.

Great Engine Protection

Excellent lubricity at hot and cold engine temperatures not only protects from surface-to-surface friction, but it also prevents the build-up of harmful deposits and acids.


Ideal middle-of-the-road oil for everyday use in all temperatures.

Benefits of 10w40 Motor Oil

Lozovoy Roman / Shutterstock.com

Added polymers

The perfect balance of the polymers with the oil ensures the rate at which the oil thickens is reduced.

High-Temperature Protection

Under 212°f testing, the oil has a viscosity of 40, this is close to the highest end of the range. This ensures the oil remains thick enough to protect the engine, even in excessively hot climates, or when it is put under increased pressure.

Good Range of Protection

Oil viscosity is measured on a range of 5 – 50. Therefore, 10 w 40 covers a great deal of the range and protects all engine components equally well at high or low temperatures.

Better by Comparison

This oil holds it's viscosity and remains thick at higher temperatures than 10 w 30 motor oil.

It is the best solution for diesel engines.

Older Vehicles

Thicker oil fills and lines any preliminary holesand cracks, creating a seal and delaying any leaks. It gives a little added protection to worn parts, the thickness is an added barrier between surfaces.

Performance Cars

High-octane cars, and those that have turbos and are highly tuned will run at higher temperatures and demand an oil that can match their performance.

The Differences Between 10w30 and 10w40

10w30 vs 10w40
Ekahardiwito / Shutterstock.com

Their first number is '10'

As both oils share the same cold weight number, their viscosities, and the way they behave in cold temperatures is virtually the same. The oil is thin enough to pump through a cold engine almost instantly, making winter starts smoother.

High-Temperature Viscosity

As the temperature rises, the polymer additive mix causes 10 w 30 oil to become thinner more slowly than 10 w 40 oil. When measured at 212°, #40 motor oil is twice as thick as #30 oil.

Everyday Driving

10w30 is ideally suited for cold weather driving that requires frequent stops and starts.

10w40 is better suited for high temperatures, whether that be from the demands of the vehicle or the climate.

Oil Types

There are two types of motor oil, both of which do a decent job of keeping the engine and all of it's moving components lubricated. However, unlike single-grade oil, multi-grade reacts differently under extreme temperatures. It behaves and does the job of two separate oils.

Single-grade Oil

As you might expect, single-grade oil behaves in the same manner, regardless if it is a freezing cold day, or you're in the height of a long, hot summer.

Until the advent of multi-grade oils, oil changes were done twice a year. The thinner winter oil was drained in spring and replaced with a thicker, more viscous oil which could withstand the summer heat.

Thin-oil left in the car throughout the summer provides inadequate protection, all surfaces don't remain protected and friction occurs. This results in premature wear and tear on metal engine components.

Single-grade oil is still widely available, but more often than not is used for lawnmowers and other such equipment.

Multi-Grade Oil

The 'W' on the packaging of your chosen motor oil denotes that it is a multi-grade oil.

They are low viscosity oils with additional polymers. These alter the properties of the oil and help it to adapt better to high temperatures.

A multi-grade oil is effective on a cold engine but is significantly better than single-grade oil on hot engines, particularly when they exceed normal parameters.

The natural thinning of the oil is slowed down, resulting in greater protection of a demanding engine and its components, especially in hot climates.

Which Oil is Best Suited for Your Vehicle?

We have ascertained that #40 oil remains thicker, for longer, at high temperatures.

Some motorists show concern that thick oil can block oil paths, and clog pipes preventing hydraulic tappets from working effectively.

This isn't true, the oil is designed to run freely through all components during extreme heat temperatures.

If you drive an older vehicle, the thick, harder viscosity of the oil provides seals where engine wear has created leakage issues. Although not a total solution, leaks can be sufficiently slowed down until repairs can be undertaken.

Grade 40 oil is equally as good in cold weather as grade 30 oil, but it performs better in warm weather.

This might make you think that it is the best oil overall, however, other factors must be considered.

For regular day-to-day driving in ambient temperatures, grade 30 motor oil is perfectly adequate, it starts working instantly on cold mornings and can make a significant improvement to fuel economy.

Final Thoughts

Driving conditions and the vehicle you own are key factors in choosing the best motor oil.

10w30 and 10w40 both operate effectively in cold conditions. However, grade 40 operates better under extreme heat due to its ability to maintain viscosity.


Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.
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