Engine Oil Viscosity Explained

Choosing engine oil may not seem like a very high priority. But selecting the right engine oil can make a significant contribution to the overall efficiency of your vehicle.

Quality is everything when it comes to maximising performance and minimising your car's wear and tear.

It is therefore crucial that the correct grade of engine oil is selected when you have your  car serviced.

If the incorrect grade of oil is used, this could:

• shorten the life of your engine
• reduce fuel economy
• create engine noise and damage overall performance
• if your car is still within the warranty then the correct grade of oil is vital in ensuring that this warranty is not invalidated

This is the reason why vehicle manufacturers recommend regular oil checks within their service schedules.

With the correct oil, wear caused by the friction of moving parts is reduced, leading to improved fuel consumption.

What do the numbers mean on oil viscosity?

When discussing the viscosity of an oil, we are talking about the measurement of the resistance to flow. 

Generally, when you are looking at engine oil gradings, you will see that there are two numbers. These help you to understand the viscosity.

You will also notice that the first number is preceded by 'W'. This stands for winter.

Telling you how the oil will flow in cold temperatures. This is important for understanding how it will behave when you start the engine.

The second number tells us how well oil will flow when an engine gets to normal operating temperatures.

A rule of thumb to follow is the smaller the number, the better the oil flows. 

Different Oil Grades Explained

A vehicle manufacturer will specify the approved grade of oil for use in their engines. Always follow the manufacturer’s recommendation.

0W-20 Oil

0W-20 is a thin, fully synthetic oil that flows easily in cold weather, helping your engine start smoothly on chilly mornings. It's ideal for modern engines that need efficiency and low emissions.

0W-30 Oil

0W-30 offers excellent cold-start performance and maintains good protection at higher temperatures. It suits many newer engines and helps improve fuel economy without sacrificing wear protection.

0W-40 Oil

0W-40 oil delivers great all-weather performance, flowing well in freezing conditions and staying strong under heat. It’s often used in high-performance or turbocharged engines.

5W-30 Oil

5W-30 is a popular, versatile oil that balances good cold starts with reliable protection during everyday driving. It’s suitable for a wide range of cars and driving styles.

5W-40 Oil

5W-40 provides better high-temperature protection than 5W-30 while still flowing well in the cold. It's a solid choice for more demanding engines or drivers who push their cars harder.

5W-50 Oil

5W-50 oil is designed for high-performance engines, offering a wide temperature range and strong protection during heavy driving. It’s great for sports cars or older engines needing a thicker oil at high heat.

10W-40 Oil

10W-40 is a thicker oil that performs well in warmer weather and under heavier loads. It's often recommended for older engines or those with higher mileage.

10W-60 Oil

10W-60 is a high-performance oil that stays stable under extreme heat, making it ideal for racing or high-speed driving. It’s typically used in sports or luxury cars that demand extra protection.

15W-40 Oil

15W-40 is a thicker oil that works well in older engines and warmer climates, offering strong wear protection. It’s commonly used in diesel engines and vehicles that carry heavy loads.

Engine Oil FAQs

Is 10W-40 or 20W-50 thicker?

20W-50 is thicker than 10W-40, especially when the engine is hot. It’s often used in older or high-mileage engines needing extra protection.

Is 10W-30 thicker than 5W-30?

Yes, 10W-30 is slightly thicker than 5W-30 when cold, but both have the same thickness at normal engine temperatures. The lower "W" number means better flow in cold starts.

Is 10W-40 good for hot weather?

Yes, 10W-40 handles heat well and keeps its thickness when the engine gets hot. It’s a solid choice for warmer climates or older engines.

Which is thicker, 15W-40 or 20W-50 viscosity?

20W-50 is thicker than 15W-40 both when cold and hot. It's better for engines under heavy load or in very warm weather.

What is more viscous, 5W-30 or 20W-50?

20W-50 is much more viscous than 5W-30, meaning it’s thicker and flows more slowly. It offers more protection in older engines or extreme conditions.

Is 30W thicker than 15W-40?

Yes and no. Straight 30W is thicker than 15W-40 when cold, but 15W-40 is more advanced, flowing better cold and staying stable when hot. It’s generally the better all-round option.

What is thicker, 5W-30 or 10W-40?

10W-40 is thicker than 5W-30, especially at higher temperatures. It’s often used when more protection is needed, like in older or hardworking engines.

Please note: always check your vehicle’s handbook to find out the recommended oil type and viscosity level for your specific vehicle. If you are unsure, contact the manufacturer or dealership to get expert advice on the right oil option for your vehicle