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The 10W40 Oil Guide (Meaning + Uses + 6 FAQs)

October 27, 2021

You’re probably familiar with 5W-30 and 5W-20 motor oils. 
These viscosity grades are commonly used in most modern passenger car engines. 

But what about 10W40 motor oil? 
Where is 10W40 oil used? 

In this article, we’ll do our best to explain 10W-40 motor oil — what the numbers mean, and where this oil is used. We’ll also go through some related FAQs, including why cars use multigrade oils.

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Let’s dive in.

What Does 10W40 Mean?

10W-40 is the viscosity, or weight, of the motor oil as defined by the Society of Automotive Engineers (SAE for short). 

A 10W-40 oil has a viscosity grade of 10W at a low temperature and 40 at a higher temperature. 

What does this mean, exactly? 
Motor oil thickens when it’s cold and becomes thinner when heated up. 10W40 engine oil doesn’t gain viscosity when it heats up. It just behaves like a 10W weight oil when cold and like a 40 weight oil at hot.

Let’s break 10W-40 down a little further.

The 10W rating: 
The 10W represents the oil’s cold viscosity. 

Oils have a specified maximum viscosity at a cold temperature. The lower the W number is (“W” stands for Winter), the thinner the oil will be. In this case, a 10W rated oil will be thicker in winter than a 5W oil. 

The 40 rating:
The 40 represents the oil’s viscosity at hot temperatures. It looks at how well the oil flows at an engine running temperature of 100oC (212oF). The hot viscosity rating focuses on seal leakage and oil’s ability to protect engine components when it’s in a thinner state.

A 40 weight oil will be thicker than a 30 weight oil at engine operating temperature. 

Now that we know what 10W-40 means, let’s see where this oil is used.

What Is 10W-40 Oil Used For? 

You won’t likely see 10W-40 as an oil recommendation on a modern-day passenger car. 

However, it still retains popularity with medium and heavy-duty gasoline engines in light trucks. This oil weight is commonly used in diesel engines or in a smaller motorcycle engine too.

The 10W-40 oil viscosity also often serves as an alternative for older engines with burning or oil leaking issues. 

Why is that? 
The 10W-40 engine oil has a thicker viscosity than, say, 10W-30 oil when the car engine is hot. This helps it lubricate older moving parts in high mileage engines while being less likely to leak. 

The thicker oil viscosity also means that it’s an excellent option for engines with high oil temperatures, as it’ll have better resistance to thermal breakdown. 

If you choose to use 10W-40 oil, synthetic 10W-40 might be a good idea for smoother start-up protection. Synthetic motor oil flows better than conventional motor oil (mineral oil) while maintaining sufficient viscosity to protect piston skirts and bearings when the temperature rises. 

Now that we know what 10W-40 oil is, how about some FAQs?

6 FAQs On 10W40 Oil

You’ll find the answers to some common questions on the 10W-40 oil here:

1. Is 10W-40 Oil Synthetic?

Like most multigrade motor oils, 10W-40 oil can be synthetic oil, semi synthetic oil, or conventional motor oil. There’s also a high-mileage variation. 

It’s important to remember that “10W-40” refers to its SAE viscosity grade, not oil type.

2. Should I Use 10W40 Or 10W30?

10W-40 and 10W-30 oils are pretty similar, though they aren’t exactly the same. 
Here are some things to consider when deciding to use one motor oil grade over the other:

A. Ambient Temperature: 

Ambient temperature doesn’t add to engine heat during operation. It does, however, influence oil viscosity. This is why your driving location is essential when picking an oil. 

The less viscous 10W-30 motor oil would run smoother in cooler regions. 
The thicker 10W-40 oil would be more efficient in preventing engine wear and tear in the higher temperature of a warmer climate.

B. Fuel Economy

10W-30 motor oil is generally more widely available than 10W-40, so it tends to be less expensive. And, because it’s less viscous than 10W-40, the engine needs less energy to pump it, so it also offers better fuel economy. 

C. Manufacturer Specifications:

For proper lubrication of internal engine parts, it’s always advisable to follow the engine manufacturer’s recommendation on oil viscosity. 

If your vehicle manufacturer doesn’t recommend 10W-30, you shouldn’t use this oil type just because it offers better fuel economy or a lower price. Using the wrong oil might affect your engine life in the long term, making it a potentially unwise trade-off. 

3. Which Is Better 5W30 Or 10W40? 

These oils have different viscosities at different temperatures. 
If your vehicle requires 10W-40 motor oil, you shouldn’t use 5W-30 oil, and vice versa. 

Here’s how they differ:

5W-30 is a thinner oil than 10W-40 and flows quicker at a cold temperature. Consequently, 5W-30 oil lubricates and protects the car engine better at a low temperature — especially during engine start-up in cold, winter weather.

A “30” high temperature viscosity grade is common (as in 5W-30, 10W-30, etc) and is suited to many engines. 

However, if you have issues with engine wear or leaks, the thicker “40” grade oil will better protect an engine at operating temperature. It also escapes leaks at a slower rate.

4. What Is Oil Weight?

Oil weight refers to the numbers in a name like “10W-40”. It doesn’t refer to how heavy the oil is but is a measure of the oil’s viscosity at specific temperatures. Alternate terms for oil weight include “oil grade” or “oil rating.”

Lower oil weight numbers generally mean a thinner oil; higher is a thicker oil. 

An engine oil’s operating temperature doesn’t change significantly, even in different ambient temperatures. However, ambient temperature plays a more significant role at engine start-up.

So, oil weights are primarily recommended based on the expected ambient temperature of an engine, and the starting temperature in particular

5. Why Do Cars Use Multigrade Oils?

Motor oil viscosity varies with temperature — thinning when hot and thickening when cold. 

A thinner oil is more useful at engine start-up as the oil can flow quickly for engine lubrication. But as the engine temperature rises, oil that’s too thin can be an issue. 

Single-grade oils (like SAE 10W or SAE 30) will be either too thick to quickly lubricate the engine at start-up, or become too thin when the engine is at a high temperature. 

This is where the multigrade oil comes in. 

A multigrade oil has long-chain polymers that contract and expand with temperature changes, altering the oil’s behavior. This characteristic allows the oil to be thin enough initially, when the car engine is cold, but retains sufficient viscosity at operating temperature. 

6. What Do Motor Oil Additives Do? 

Oil manufacturers use viscosity index improver additives to achieve temperature-specific viscosity grades. These additives allow the engine oil to act like a thinner oil at a cold temperature and be like a thicker oil when it’s hot. 

Additives don’t just help control the lubricating properties of an oil. They also have the significant task of managing engine wear and contaminants. 

Additives help break down piston deposits, have dispersants to prevent sludge formation, and corrosion inhibitors to prevent rusting of metal surfaces. 

But there’s a caveat. 

Additive packages are constrained by catalytic converters and emissions warranty requirements. Ingredients like zinc, phosphorus, and sulfur in additives help prevent camshaft wear. But these elements can contaminate precious metals in a catalytic converter. 

As such, the amount of substances in additives that can damage catalytic converters must be limited to ensure that catalytic converters last to the end of their warranty. 

Closing Thoughts

Using the correct oil viscosity grade for your gasoline or diesel engine is crucial to ensuring its longevity, regardless of whether you drive in temperate climates or extreme temperatures.

But in an emergency, any oil is still better than no oil, whether it’s 10W-40 or otherwise. 

Just make sure to visit your mechanic afterward to flush out the wrong oil and put the right one in. Don’t forget to change your oil regularly, too, as sludge will form and it’ll become ineffective.

If you need help with an oil change or any issues with your car, your easiest option is a mobile mechanic. This way, you don’t have to drive your vehicle to a workshop. 

For that, you have RepairSmith

RepairSmith is a mobile vehicle repair and maintenance solution, available seven days a week. Just contact them, and their ASE-certified technicians will drop by to help you out in no time!