How do we read the markings on motor oil packaging (5W-30, 10W-40, etc.)?

The markings on motor oil packaging show how it behaves at low and high temperatures and whether it is suitable for a particular engine. Correct interpretation is key for optimal lubrication, reduced wear and a longer service life of the engine.

In the following lines you will get a clear and practical answer about what codes like 5W-30 and 10W-40 mean, how they are defined by international standards, and how to use them when choosing oil for your car.

What do the markings on motor oil mean?

Motor oil markings are standardized to provide unambiguous information about the oil's viscosity and behavior at different temperatures. In most cases these are all-season oils designed to perform reliably both at cold start and when the engine is fully warmed up.

These characteristics are defined by international norms, the most common viscosity standard being that of the SAE.

What is the SAE standard and what is viscosity?

SAE (Society of Automotive Engineers) is an international organization that creates standards for the automotive industry. One of them is SAE J300, which classifies motor oils according to their viscosity.

Viscosity describes a fluid's resistance to flow. For motor oils this means:

• how quickly the oil flows at low temperatures;
• how stable the oil film remains at high operating temperature.

It is important to emphasize that viscosity is not a constant. It changes with temperature, and the SAE standard sets precise limits within which an oil must fall to carry a given designation.

How to interpret designations like 5W-30, 10W-40 and similar?

All-season oils are marked with a combination of numbers and a letter that always follow the same logic. To make it clearer, let's examine their structure step by step:

What does the first number before the letter W indicate?

The first number determines the oil's viscosity at low temperatures. The lower this number, the more fluid the oil remains in cold weather and the easier it reaches all moving parts of the engine at startup.

Here is what the most common values mean:

• 0W – excellent fluidity in extreme cold;
• 5W – very good protection in winter conditions;
• 10W – suitable for moderate climates and older engines.

A lower number means less wear at cold start, when the engine is most vulnerable.

What does the letter W mean?

The letter W comes from the English word Winter. It indicates the oil's ability to retain the necessary fluidity at low temperatures.

This is particularly important because a significant portion of engine wear occurs in the first seconds after ignition, before the oil has reached all components.

What does the second number after the dash indicate?

The second number indicates the oil's viscosity at the engine's operating temperature, usually around 100°C. It determines how resilient the oil film is under high load and prolonged operation.

For example:

• 30 – lower viscosity, characteristic of modern, fuel-efficient engines;
• 40 – higher protection under load and at higher temperatures;
• 50 – used in sporty or heavily loaded engines.

This number is crucial for protecting the engine during long drives and high RPMs.

What is the difference between 5W-30 and 10W-40?

The difference between these two oils lies both in their behavior at cold start and at operating temperature. The main differences can be summarized as follows:

• 5W-30 provides faster lubrication at cold start and lower internal resistance – it is preferred for modern engines with tight tolerances.
• 10W-40 is thicker at low temperatures and maintains a thicker oil film under high load – often used in older cars or high-mileage engines.

It becomes clear that the choice is not a matter of "better" or "worse" oil, but of compatibility with the specific engine.

Understanding the markings on motor oil gives a real advantage in vehicle maintenance. The right choice leads to less wear, better efficiency and a longer engine life.