Understanding Motor Oil Grades for Optimal Engine Health

19 Mar 2025

Choosing the right engine oil can be difficult if you don’t know which motor oil you need and don’t understand what the different oil grades mean. The right engine oil for your vehicle will be determined by your vehicle’s manufacturer but it can also be influenced by the age of your vehicle, how often you drive it and the climate you are driving in. To help you understand more about motor oil grades, we have answered the following questions:

• What does engine oil do?
• Engine oil grades explained
• What do the engine oil grades mean?
• Can you mix engine oil grades?
• What is the viscosity index?
• Types of engine oils

What does engine oil do?

Engine oil, also referred to as motor oil, maintains your vehicle's engine's efficiency by reducing friction between moving parts. It lubricates, cools, cleans and protects the engine to keep it working smoothly. This can help to reduce the wear, tear and corrosion of engine parts, improve fuel economy and prolong the life of your vehicle’s engine so it’s important to ensure your engine oil is appropriately topped up. Without motor oil, your engine could seize up, overheat, and lead to significant damage that could result in the entire engine being replaced.

Engine oil grades explained

Before you buy engine oil for your car, it’s important to check your vehicle owner’s manual for the approved engine oil grades list. It can also be extremely useful to understand what the engine oil classifications mean so you know you are using the right engine oil for your vehicle.

Single-grade engine oil

Single-grade engine oils don’t adapt to temperature changes & perform at a fixed viscosity. This means they can perform in hot or cold temperatures but they don’t adapt well to fluctuating climates.

Single-grade engine oil will usually be displayed with a single number. For example: SAE 30.

Multi-grade engine oil

Meanwhile, multi-grade engine oils are better suited to the longevity of modern vehicles and are designed to perform across different temperature ranges. This is the engine oil you’ll come across most often and is displayed with two numbers and a letter. For example: 10W-30.

But what does it mean?

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What do the engine oil grades mean?

Engine oil viscosity is also referred to as the Society of Automotive Engineers or SAE viscosity grade. This is displayed as different numbers for different oils, which can look confusing at first. So, we want to help you understand what it all means. The grading of oil will look something like this: 10W-30.

The number (10) before the W refers to how thick the oil is and how well it flows in colder climates or when you first start the engine.

The W stands for winter and indicates how well the oil flows in cold conditions.

The number after the W tells you how well the oil flows in hot or warmer conditions.

To understand how it works, it’s useful to think about how oil reacts when you first put it into a pan to heat. When the oil is at a lower temperature it is much thicker but as it heats up, it starts to get thinner and more runny. That’s exactly what happens in your vehicle too.

The lower the first number, the easier the oil will flow in colder temperatures and the higher the number, the thicker the oil.

As an example, a 10W30 oil will flow better in colder climates and when you first start your engine whereas a 20W50 will offer better protection in hotter climates.

Can you mix engine oil grades?

Even though all engine oils are made to SAE standards, you shouldn’t mix engine oil grades or even different brands. That’s because different brands use their own formula so using the wrong oil could compromise the protective properties of your oil.

If you do want to use a different brand of engine oil, you should do this after an oil change and engine flush to remove any sludge.

What is the engine oil viscosity index?

Viscosity is  the thickness of the oil. The viscosity index is how temperature influences the oil viscosity and usually, the higher the temperature, the lower the viscosity. By understanding viscosity, you can ensure you are using the right engine oil for your engine’s operating temperatures and requirements.

Types of engine oils

There are four main types of engine oil including synthetic, semi-synthetic, high-mileage and conventional oil. Each will be suited to different vehicles. Consult your vehicle engine manufacturer’s handbook or guidelines about what engine oil is best for you.

Synthetic oil

Synthetic oil is a man-made lubricant that uses artificial chemical compounds. This typically includes a base oil like distilled crude oil and chemically modified petroleum components. Synthetic oils are commonly used in high-performance vehicles and vehicles that are suited to extended oil change intervals.

Semi-synthetic oil

Semi-synthetic oil blends synthetic and mineral oil. It offers better performance than mineral oil but is cheaper than fully synthetic oil so can be a more affordable choice. Semi-synthetic blends are ideal for vehicles that are used for short trips or aren’t used often.

Conventional oil

Conventional oil, or mineral oil is derived from crude oil which is then refined. It is often better suited to older vehicles as it’s thicker but it can often struggle with temperature fluctuations and requires frequent oil changes.

To determine what engine oil you need, you should consult your vehicle manual. The right engine oil can prolong your car, motorcycle or commercial vehicle and affect overall performance. It’s important to understand the different engine oil grades and types of engine oil so you can choose the right motor oil for your vehicle and reap the benefits of optimal engine performance for many years to come.