What does Engine Oil Numbers Mean?

What does Engine Oil Numbers Mean?

Ever been puzzled by motor oil labels like SAE 5W-30 or 15W-40?

These numbers indicate oil thickness, with lower numbers being thinner.

The “W” means winter, indicating oil maintains viscosity in cold temperatures.

Different grades matter based on weather changes affecting oil thickness.

An image illustration of What does Engine Oil Numbers Mean
What does Engine Oil Numbers Mean
Source: (Freepik)

The Engine Oil Numbers

The Society of Automotive Engineers (SAE) standardizes engine oil viscosity to ensure global consistency.

Initially, there was a single number like SAE 30, but due to temperature variations, they introduced multi-grade numbers like 15W-40.

These numbers indicate the oil’s performance in cold and hot conditions.

A 15W-40 oil remains the same across brands and countries, with gradations for both cold and hot climates based on flow tests at different temperatures.

The oil grades reflect how well the oil performs in specific temperature ranges, impacting engine performance.

Understanding Engine Oil Numbers

Multi-grade oils have specific viscosity values based on scientific measures.

In the cold, cranking viscosity and pumping viscosity indicate oil behavior during engine start.

In the heat, kinematic viscosity and shear rate measure flow and movement speed.

Don’t get bogged down by technical details; the key takeaway is the meaning of grade numbers.

For instance, 5W-30 is thinner than 20W-50, aligning with a trend for thinner oils.

Thicker oils increase friction, impacting fuel economy, prompting a shift to thinner oils, such as 5W-30, to achieve better fuel efficiency.

An image illustration of Engine Oil Numbers Meaning
Engine Oil Numbers Meaning
Source: (Freepik)

Oil Viscosity Explained

Viscosity, or oil thickness, varies with temperature.

Cold makes oil thicker like honey, while heat thins it like water.

This property is known as engine oil weight— heavier means thicker, lighter means thinner.

Diverse viscosities serve to protect engines in extreme temperatures, from freezing mornings to hot days pulling heavy loads.

Engine designers consider viscosity for optimal lubrication, preventing premature wear between moving metal parts with gaps or tolerances.

Engine seals are designed for specific oil viscosity, emphasizing the importance of following manufacturer recommendations based on climate.

Variations exist, such as different viscosity recommendations for a hot desert in Egypt versus the cold wastes of Alaska.


Understanding engine oil numbers is crucial for maintaining optimal engine performance.

These numbers, determined by organizations like the Society of Automotive Engineers (SAE), signify the oil’s viscosity or thickness at different temperatures.

The multi-grade system, such as 5W-30 or 15W-40, reflects the oil’s ability to flow in both cold and hot conditions.

The viscosity impacts lubrication, protecting the engine from wear and ensuring proper functioning.

By grasping these numbers and their significance, vehicle owners can make informed choices.

Aligning the oil’s properties with their specific driving conditions and the manufacturer’s recommendations for enhanced engine longevity and efficiency.


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