5w40 vs 15w40: Which Is The Better Motor Oil?

It might be difficult to choose the proper motor oil for your engine. 5w40 and 15w40 are two great motor oils. Many folks mix them up and think they’re the same thing. Well, those two aren’t the same thing to begin with. 

Worry no more because today we’ll clear the confusion!

So, what is the difference between 5w40 vs 15w40?

Fuel efficiency and shear stability are almost the same with the two engine oils. But the 5w40 performs slightly better at them. Although it does well only in the cold, it performs better than 15w40. However, 15w40 can be used in hot weather as well.

This is just the TLDR of all the differences between the two. Interested in knowing all those? Scroll down and read the rest of the article. 

5w40 Vs 15w40: Key Differences

In order to distinguish between 5w40 and 15w40, it’s important to know their features. So before we get into the detailed differences, let’s look at some of the notable ones first. They should give you a better understanding if you should take 5w40 instead of 15w40!

Fully synthetic motor oilSynthetic blend engine oil
Sludge and deposits are combated with additional detergentsSuperior Deposit Control and Cleanliness
5 degrees is the maximum working temperature of 5w40 engine oil15 degrees is the lowest temperature that 15w40 motor oil can run at
Extra Wear Protection To Make Engine Parts Stay NewWith Triple Protection Plus Technology, it protects against wear
Great for using in winter Does moderately good in winter
Starts quickly and provides fast protection in winter or cold weatherGood oil flow in low temperature
Increased oxidation and thermal stabilityThermal stability and high oxidation

5w40 Vs 15w40: Detailed Comparison

Still, thinking about what to choose between 15w40 and 5w40? We’ll dive into the specifics now so you can gain a better understanding. Let’s not spend any more time and get down to business-

Fuel Efficiency

The 5w40 is more fuel-efficient than the 15w40. It lowers the engine’s consumption of fuel and improves the fuel-efficiency. You can get a 1.5% fuel efficiency better than the 15w40 motor oil with the 5w40.

The compatibility of the emission system is better with the 5w40. This oil is produced with an advanced formula called low-ash.

This formula helps the oil in controlling the exhaust block once it’s done using treatment. And this makes it easy for the vehicle’s emission compliance to be maintained. Not to mention, this lets you get an increased fuel economy.

Also, the oxidation resistance of the fuel increases fairly. This allows it to manage the viscosity of the oil. The fuel efficiency of the oil is enhanced thanks to the improved oil drain capability. 


The 15w40 engine oil comes with lower volatility. This is all thanks to the dispersant additives and the detergents available in the oil.

Lower volatility in the oil can decrease fuel consumption. Just like the 5w40, this engine oil also has pretty good oxidation resistance. Letting it control the oil’s viscosity better. The drain capability of the fuel is good with this oil as well. 

Although both oils can be seen to be on par, the 5w40 does everything slightly better. 

Extreme Weather Performance

As an engine oil, comparing their performance in extreme temperatures is a must. The 5w40 performs perfectly fine in cold weather.

Even when your engine is too cold, it’ll still perform just fine. This is due to the lesser viscosity of the oil. Due to the lower viscosity, it doesn’t take time to start the engine.

This will lubricate the engine pretty well from the inside. Making it easy for the oil to circulate inside the engine. 

The 15w40 performs pretty well in both hot and cold. This is one of the finest aspects of this engine oil.

The oil’s higher viscosity keeps it from becoming too liquidy. But, the viscosity is not so high that you can’t use it in the cold. It works perfectly fine in the cold too. And you can use this oil on all engines starting from modern ones to old ones. 

Shear Stability

The shear stability of the oil is one to talk about. The 5w40 oil provides great shear stability. This keeps the oil safe from viscosity loss and keeps the engine’s oil pressure stable.

The shear stability of the 15w40 oil is also very good. It does everything the 5w40 does except the 5w40 does them better. The 5w40’s shear stability is better than the 15w40. 

Can I Use 15w40 Instead of 5w40?

No, we don’t suggest using 15w40 instead of 5w40. Even though they both perform almost the same, the 5w40 gives slightly better performance.

When it comes to fuel efficiency or fuel stability, they both perform almost exactly the same. But, 5w40 performs slightly better.

But in terms of weather, 5w40 works really well in the cold. But 15w40 works well in both hot and cold. It doesn’t do better than 5w40 in the cold though.

So, the 5w40 engine oil is the winner!

What Happens If I Put 5w40 Instead of 15w40?

Well, firstly, if your car engine needs an oil type like 15w40, you should stick to it. Otherwise, putting 5w40 shouldn’t be a problem. Because this oil performs really well in extremely cold weather.

But in hotter weather, 15w40 performs better than 5w40. So, if you live in a region that stays hot mostly, putting 5w40 instead of 15w40 will be harmful. But in colder weather, your engine will be more efficient, resulting in better car performance. 

Can I Mix 5w40 and 15w40?

Yes, you can mix 5w40 with 15w40. The two oil types are quite compatible. So, blending them together shouldn’t be problematic for your car.

Instead, with a 50/50 mixture of 5w40 and 15w40, you’ll have a semi blend of 5w40. This new oil should provide excellent performance in both hot and cold weather.


Question: Can I use a 5w40 with diesel engines?

Answer: You can use the 5w40 with almost any type of engine. However, you must purchase the type-specific oil for that. E.g. for diesel engines, you have to use the diesel one.

Question: Why do diesel engines use 15w40 engine oil?

Answer: Diesel engines usually combust at a much greater compression compared to gasoline ones. Since these engine oils tend to be denser and heavier.

Question: When should I use 5w40 oils?

Answer: It’s best to use these oils when at colder temperatures as they mix well in cold. So it’s better to mix them when you haven’t used your car for hours. That is until your engine has cooled down.


This is all there’s to know about 5w40 vs 15w40. Both are great engine oils. 5w40 is the ideal option to go with here. But if you live somewhere it’s hot most of the year, go with 15w40. 

Please let us know if you have any other questions in the comments section.

Good Luck!

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