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10W30 vs 10W40 – what’s the difference between the two oil grades? Do you even need to care?
If you are looking to replace oil in your car, then you’ve probably come across 10W30 and 10W40 oil grades – perhaps the mot common oil grades available on the market.
And due to this, the vast majority of car owners at some point start wondering what the differences are between the two.
Things are simpler than you may be thinking. However, before we explain to you what the differences between these two types of oil are, we need some background information.
10W30 Oil vs 10W40 Motor Oil – What’s The Difference?
Short answer – 10W30 oil is less viscous (thinner) vs 10W40 oil at high motor temperatures. In cold temperatures, their viscosity should be nearly identical.
The difference between the viscosity of the two oil types at high temperatures is apparent from the second number in the designation – 30 vs 40.
With that said, what do these numbers truly mean? And why should you care about the viscosity of motor oil at different temperatures?
Why Oil Viscosity Matters In Car Motors
The purpose of motor oil is to lubricate the engine’s internal components, protecting them from friction and heat. Apart from that, engine oils have a cleaning function – they eliminate sludge and varnish from the motor.
To be able to perform its functions, engine oil needs to be viscous enough, but not too viscous.
If you don’t remember this from the physics class, viscosity is the resistance of a fluid to deformation. A fluid with high viscosity is thicker, is more resistant to movement, and thus flows less easily. In contrast, a fluid with low viscosity is thinner and flows more easily and is waterier.
A perfect example of a low-viscosity fluid is water – it is one of the least viscous fluids in existence. Oils or gels, on the other hand, are much more viscous than water.
Now that we’ve got this part refreshed, let’s try to understand why balanced oil viscosity is crucial for safe motor operation:
- If the oil is too viscous, then it will not flow easily enough through the motor to sufficiently lubricate it and protect it from wear.
- If the oil is too watery, then it will not be able to form a lubricating film between motor surfaces, and friction will reappear.
Next, what does this all have to do with temperature?
At low temperatures, motor oils become more viscous and flow harder. However, in cold, the oil should stay thin enough to be able to flow with a sufficient rate to lubricate the motor.
At high temperatures, motor oils become waterier and less viscous. But the oil should stay viscous enough to be able to separate engine surfaces from each other.
When it comes to our 10W30 vs 10W40 comparison, the viscosity of the oils will be slightly different as the motor temperature changes. These differences are exactly the reasons why you should carefully consider oil types and understand the differences between them.
How To Read Oil Viscosity Grades
Now, let’s try to understand what exactly the numbers in oil viscosity ratings mean.
The first number along with the letter “W” refers to the performance of the oil at low temperatures. The second number refers to the performance of the oil at high temperatures. Let’s talk about each more in-depth.
Oil cold-temperature performance
10W denotes the motor oil’s changes in viscosity at low temperatures. The “W” in the rating stands for “Winter”. In the case of our 10W40 vs 10W30 comparison, both oils will perform identically at low temperatures.
If the cold-temperature number was lower – say, 5W – the oil would be able to stay sufficiently viscous at lower temperatures than 10W oil. 15w Wil, in contrast, would become too viscous for normal operation at similarly cold temperatures.
In other words, 5W oil can be safely used at lower temperatures than 10W and 15W oil.
Cold-temperature viscosity grades go from 0W to 25W, so 10W oil will operate fine at pretty low temperatures. It’s not the best for freezing temperatures though.
Oil warm-temperature performance
The second number refers to the oil’s viscosity changes at high temperatures, and here’s where the two oil types differ significantly.
For the warm-temp rating, the oil viscosity is measured at a temperature of 212 degrees Fahrenheit (100 degrees Celsius). The higher this rating, the more viscous the oil is at 212 degrees. In other words, the higher the warm-temp rating, the hotter the oil can get before becoming too watery for safe operation.
At 212 degrees, the oil with the 10W40 rating will be more viscous than 10W30 oil. Furthermore, it will be more resistant to becoming thin as operational temperatures increase. At some high temperature where 10W40 will stay viscous enough to allow for safe engine operation, the 10W30 oil will become too watery.
SAE Viscosity Grades For Engine Oils – SAE J300 Dec 99
|SAE Viscosity Grade||High Shear Rate Viscosity @ 150 Degrees Celsius (mPa.s)||Maximum Viscosity @100 Degrees Celsius (mm 2/s)||Minimum Viscosity @ 100 Degrees Celsius (mm 2/s)|
|25W||Data unavailable||Data unavailable||9.3|
|20W||Data unavailable||Data unavailable||5.6|
|15W||Data unavailable||Data unavailable||5.6|
|10W||Data unavailable||Data unavailable||4.1|
|5W||Data unavailable||Data unavailable||3.8|
|0W||Data unavailable||Data unavailable||3.8|
Oil Viscosity Grade And Fuel Efficiency
Apart from performance in high and low temperatures, the oil viscosity grade can actually impact fuel economy as well.
According to Mobil, lower-viscosity oils can improve the fuel efficiency of your car by reducing engine friction.
Since the engine has to work less to operate with a thinner oil, it will require less fuel and energy.
However, also remember that less viscous oil will likely provide less lubrication to your motor components, especially at high temperatures.
You will have to find the right balance between fuel efficiency and motor lubrication.
In our opinion, motor lubrication is more important than fuel efficiency in the long term.
Although thinner oils may allow you to save money for some time at the right temperatures, they are more likely to lead to overheating and engine damage at temperatures they aren’t designed for.
And if damage does happen, then repair or replacement costs may exceed any type of savings you might have had with low-viscosity oil, even over the course of years and decades.
Although this won’t necessarily happen, the risk is too great, and so are the costs if something goes wrong.
What About Motor Oil Formulation?
You may also be wondering whether there are any differences between the formulations of 10W30 and 10W40 motor oils. E.g. maybe 10W30 are available only as synthetic blends and 10W40 are full synthetics?
The answer is that both blends are available in a variety of formulations. Whether you are looking for high-mileage, conventional, synthetic blend, or full synthetic oils, both grades have a wide range of formulations to offer.
10W30 or 10W40 Oil – Which One Should You Choose?
Now, which oil type should you choose for your car?
Based on what we’ve talked about above:
- 10W30 oil is a good choice for areas with moderately high temperatures.
- 10W40 oil is a good choice for areas with high temperatures – as mentioned above, this oil will be more resistant to thinning as temperatures increase. Apart from that, 10W40 engine oil may be used in the summer months.
However, your car’s engine will also matter when choosing motor oil. So although we’ve established many things about oil viscosity, we cannot give you tips specific to your car because each car motor is different. Some motors operate hotter than others.
With that, the best tip that we could give you is that you should check your car’s manual – it should have instructions on which type of oil to use and when.
And as mentioned above, unless you are 100% sure that a less viscous oil will work for your car’s engine, do not attempt to save money by opting for thinner oil. As explained above, this will likely end with damaged engine components and hefty repair bills.
Our 10W30 & 10W40 Motor Oil Suggestions
Now, Garage Chief experts will suggest a few great 10W30 and 10W40 engine oil options. For the right engine and the right usage scenarios, any of our picks should work excellently.
However, please do make sure to check out your car’s manual to find out what kind of oil it needs.
Best 10W30 Oil Suggestions
Valvoline High-Mileage Synthetic Blend Motor Oil
If your car’s mileage is over 75,000 miles, then think about getting high-mileage oil like this synthetic blend oil from Valvoline. This oil is specifically formulated to compensate for the physical wear of aging engines and somewhat slow down their further degradation.
Perhaps the biggest issue in aging engines is leaks caused by hardening seals. Valvoline’s motor oil contains seal conditioners to recover engine seals to allow the oil to reach where it needs to be.
Next, this oil contains antioxidants to prevent oil breakdown and the formation of sludge in the engine. If sludge does form, then the detergents in the Valvoline oil will be able to bond with it and clean it away.
As a synthetic blend oil, the Valvoline high-mileage oil is a good option for medium-duty cars. Synthetic blends generally are formulated to provide protection at heavier loads, but they aren’t as heavy-duty as fully synthetic oils.
Notably, this particular 10W30 motor oil is among the more popular picks on our roundup. As of this post’s writing, it had over 2k ratings on Amazon along with 4.8 stars overall. So this oil seems to have worked for the vast majority of buyers with great results.
However, many people have complained about leaky packaging, but this is probably more an issue with shipping than the product itself.
Castrol GTX Conventional Motor Oil
Conventional oils like this Castrol GTX are a standard choice for new or low-mileage cars that are intended for light use. For most readers, Castrol GTX should be the right oil.
Although the Castrol GTX conventional motor oil isn’t formulated for high-mileage cars, it offers similar benefits to Valvoline’s high-mileage motor oil we’ve just reviewed. More precisely, Castrol GTX is advertised to be able to remove old sludge, as well as prevent the formation of new deposits.
The Castrol GTX 10W30 motor oil also contains anti-wear additives along with premium-quality base oils to reduce wear and increase the longevity of your car’s engine.
In terms of buyer feedback, Castrol GTX seems a little bit better than Valvoline’s high-mileage oil. Castrol GTX is equally highly rated but doesn’t appear to have issues with leaky bottles.
Castrol EDGE Advanced Full Synthetic Motor Oil
As a full synthetic oil, Castrol’s EDGE Advanced motor oil is pretty pricey. However, for heavy-duty applications, it is one of the best options available out there.
The highlight of the EDGE Advanced oil is the so-called fluid titanium technology. With this tech, the oil behaves differently under high pressure than typical motor oils. As claimed by Castrol, this oil is stronger than other full synthetic oils (namely, 3 times stronger than the “leading full synthetic” oil) at high pressures.
Not only that, but this oil is formulated to keep friction low at extreme pressures, which translates to improved longevity under heavy loads and in demanding motors.
Apart from these, the Castrol EDGE Advanced oil has anti-wear and deposit control capabilities as well – just like the previous two 10W30 oil picks.
But do you need all the high-pressure benefits offered by the EDGE Advanced oil? Most likely, you do not. As mentioned earlier, full synthetic oils are best for heavy use cases. And since they are expensive, you would be wasting money if you were to buy them for occasional driving.
Besides, this oil may lack some features that light-duty cars need, so again, do make sure that you know what your car truly needs.
Best 10W40 Oil Suggestions
Liqui Moly MoS2 Anti-Friction Motor Oil
The most noteworthy thing about Liqui Moly’s MoS2 motor oil is that it is designed for use with either gasoline or diesel motors. All other motor oils on our list (no matter the grade) are formulated for gasoline engines only.
Apart from that, Liqui Moly advertises that MoS2 can be used on engines with or without turbocharging. The MoS2 oil is also claimed to be perfectly suited to old/high-mileage engines. With all this in mind, Liqui Moly MoS2 offers remarkable versatility and can work with a wide range of motor types.
Being semi-synthetic motor oil, MoS2 is capable of effectively lubricating car motors and reducing friction at high pressures and high temperatures. All in all, MoS2 should be a great choice for heavier use cases.
And by the way, MoS2 in the product name stands for molybdenum disulfide (chemical formula MoS2). This substance has a low coefficient of friction, as well as high stability and lubricity at very high temperatures. But keep in mind that MoS2 imparts a gray tint to this motor oil.
Note that Liqui Moly’s MoS2 motor oil is a pretty expensive motor oil. Perhaps due to this, MoS2 has under 100 buyer ratings on Amazon. With that said, buyers have been highly satisfied with this motor oil, and given that its features are what your car’s motor needs, maybe it is worth the money.
Valvoline High-Mileage Synthetic Blend Motor Oil
This motor oil is nearly identical to the Valvoline high-mileage oil reviewed in the previous section. But needless to say, the two Valvoline oils have different grades.
With the 10W40 grade, this high-mileage oil should be a better choice for warm climates.
Other than that, the use cases for the 10W40 Valvoline high-mileage oil are the same. It is again formulated for cars with 75,000+ mileage and contains seal conditioners to stop leaks, detergents to remove sludge, antioxidants to prevent oil breakdown, and additives for wear reduction.
Being merely an oil style under the same listing as the 10W30 high-mileage oil, this motor oil has similar complaints as well. Most importantly, many people have encountered leaky bottles.
Mobil 1 High-Mileage Full Synthetic Motor Oil
Finally, we have this 10W40 high-mileage motor oil from Mobil. As a high-mileage oil, this motor oil is in many ways similar to the reviewed Valvoline oils, but it has a crucial difference – it is a full synthetic oil.
As such, the Mobil 1 oil will be a better choice for heavy use cases where you need maximum performance and durability. More specifically, Mobil recommends this oil for light vans, trucks, and SUVs. But, of course, you should check your car’s manual to determine whether it truly needs this sort of motor oil.
Like the overviewed Valvoline oils, Mobil 1 oil is formulated for cars with 75,000+ miles mileage. And similarly to Valvoline high-mileage oils, Mobil 1 contains a seal conditioner to treat oil leaks, provides wear protection, removes sludge, and prevents its further formation.
As a full synthetic motor oil, Mobil 1 high-mileage oil is pretty pricey. However, buyers seem to have been very satisfied with this product, so if it’s indeed the right oil for your car, then it should be worth the money.
10w30 vs 10w40 – Final Words
Hopefully, our guide was able to help you with picking the right motor oil for your car.
Remember that when it comes to the 10W30 vs 10W40 debate, what matters is the ability of the motor oil to resist thinning at high temperatures. Performance or fuel efficiency gains aren’t as important as this.
So when shopping for motor oil, consider temperatures in your area, as well as the recommended oil type for your car.
Last update on 2021-02-26 / Affiliate links / Images from Amazon Product Advertising API
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