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Which oil is better, 5W-30 or 5W-40?

If you are wondering which oil is better, 5W-30 or 5W-40, there is no single simple answer Both are multi-grade lubricants designed to protect the engine under various temperature conditions, but their performance changes once the engine reaches operating temperature.

The choice depends not only on the number on the bottle, but also on the engine design, internal tolerances, and manufacturer's specifications. Driving style, climate and vehicle technology also play a key role, especially in modern engines with emission control systems.

Generally speaking, 5W-30 is thinner at high temperatures, while 5W-40 maintains higher viscosity. However, this does not mean that one is universally better than the other. Using the wrong oil can affect engine lubrication, efficiency and protection.

Therefore, before choosing between the two, it is essential to consult the vehicle's manual and use a product that meets the required specifications. In the Repsol Lubricants product range, for example, you can find options such as Repsol Elite 5W-30 or Repsol Elite 5W-40, formulated for different engine needs, meeting specific standards.

Differences between 5W-30 and 5W-40: what these numbers mean

The designations 5W-30 and 5W-40 belong to the SAE viscosity rating, which describes how oil flows at different temperatures.

The first number, accompanied by the letter W (winter), indicates how the oil performs in cold conditions. The lower this value, the more easily the lubricant flows during start-up.

The second number represents the oil’s viscosity when the engine is warm. This is where the main difference lies:

5W-30: lower viscosity at operating temperature.
5W-40: higher viscosity at operating temperature.

Both share the same cold rating, so their startup performance is similar. However, when the engine reaches normal operating temperature, 5W-40 maintains a thicker oil film.

This does not mean one is better than the other, but rather that they are designed for engines with different requirements.

Feature	5W-30	5W-40
Cold Viscosity	5W	5W
Fluidity at startup	Similar	Similar
Warm viscosity	Lower	Significant
Film strength	Suitable for optimized engines	Greater in this comparison
Potential efficiency	Higher in engines designed for it	Lower relative fluidity
Application	As specified by manufacturer	As specified by manufacturer

Which oil is more viscous, 5W-30 or 5W-40, and why does it matter?

At operating temperature, 5W-40 is more viscous than 5W-30. This difference affects how oil flows through the engine and how the protective oil film forms between metal parts.

The oil must fulfill two key functions:

Flow quickly to lubricate all components.
Maintain a stable oil film that reduces metal-to-metal contact.

Being thinner at operating temperatures, 5W-30 can improve fuel efficiency in engines designed for this viscosity grade. Being more viscous, 5W-40 can provide a stronger oil film for engines that require it.

This difference affects:

Oil fluidity: 5W-30 flows more easily.
High temperature protection: 5W-40 maintains a thicker oil film.
Fuel consumption: may vary depending on engine design.
Compatibility with modern systems: such as turbochargers or variable valve timing.

In the Repsol Lubricants range, products such as Repsol Elite Evolution DX2 5W-30

Repsol Elite Evolution 5W-30 is formulated for engines focused on efficiency, while Repsol Elite Multiválulas 10W-40 is geared towards more demanding temperature and load conditions.

Which is better based on your engine and driving conditions?

The choice between 5W-30 or 5W-40 oil depends primarily on the engine, not on vehicle mileage or age.

The most important factors are:

Engine design and internal tolerances.
Type of lubrication system.
Engine technology (turbo, direct injection, etc.).
Typical operating conditions.
Manufacturer approvals.

Modern engines often require very specific lubricants. That is why products in the Repsol Elite line are developed to meet various standards and adapt to different technologies.

Driving conditions also matter. Urban driving with frequent stops does not put the same stress as highway driving or operating under heavy loads. However, this does not justify changing viscosity without following the manufacturer's guidelines.

When to choose 5W-30?

5W-30 is suitable when expressly recommended by the manufacturer and the product meets the engine manufacturer specifications.

It is tipically used in:

Modern gasoline and diesel engines.
Engines designed for maximun efficiency.

An example is Repsol Elite 5W-30, formulated to provide engine protection and cleanliness in engines that require this viscosity grade.

When to choose 5W-40?

The 5W-40 is used in engines that require higher viscosity at operating temperature or when recommended by the manufacturer.

It may be suitable for:

Engines that operate at high temperatures.
Vehicles with higher mechanical demands.
Applications where higher lubricant stability is required.

In these scenarios, products such as Repsol Elite 5W-40 offer a formulation tailored to these conditions.

How do you know what oil is in your car?

Before choosing between 5W-30 and 5W-40, always follow these recommendations:

Refer to your vehicle's manual.
Check the recommended viscosity.
Check the required specifications.
Check the maintenance history.
Use manufacturer or lubricant oil-finder tools.
Consult a professional if in doubt.

The same car model can have different engines, so not all of them use the same oil.

What happens if you mix 5W-30 and 5W-40?

Ocasionally mixing both oils will not usually cause immediate failure, but it does alter the final viscosity and can affect lubricant performance.

This practice should be avoided in all cases. When topping up oil, it is best to use a product compatible with engine specifications, such as lubricants from the Repsol Elite range, which cover different viscosity and approval needs.

In short, there is no universally better oil. 5W-30 and 5W-40 are designed for different engines, and the choice depends on the manufacturer's specifications.

5W-30 offers greater fluidity at high temperatures, while 5W-40 provides higher viscosity under those conditions. Both may be suitable if they meet vehicle approvals.

The key is not to choose the most viscous or the most fluid oil, but the right one for your engine. To do this, always relies on the vehicle's manual and specifically formulated lubricants, such as those in the Repsol Elite range, developed to offer protection, efficiency and compatibility with current technologies.