Engine Oil Grades And Engine Oil Viscosity Explained

Choosing the correct engine oil for your vehicle can help it run smoothly. But given all the options for engine oils in the market, choosing one can be a daunting task. Determining the best engine oil for your car depends on several factors, like the climate you live in, your driving habits, engine type, manufacturer's specifications, etc. While there is a mountain of information about the types of engine oils used in vehicles, there are some crucial points you need to know before selecting one. This article will help you learn about different engine oil grades, the meaning of engine oil grade, viscosity, and how to choose the correct engine oil for your vehicle.

In this article:

What Is The Viscosity of Engine Oil?

Viscosity is the measure of a fluid's resistance to flow. Water, for example, flows ‌quickly because it has low viscosity, and honey flows slowly because it has a comparatively higher viscosity. Similarly, engine oil with higher viscosity will be thick and flow slowly, whereas oil with low viscosity will be thin and flow easily with minimal resistance. 

Engine oil viscosity is the most crucial feature because it determines how efficiently it will lubricate the engine and its components at different temperatures. The viscosity of a fluid is inversely proportional to the temperature, i.e., when the temperature rises, the viscosity decreases, and when the temperature drops, the viscosity increases. The main cause of change in engine oil viscosity is engine temperature. Oil gets thinner when the engine is hot and thicker when it's cold.

What Do Oil Viscosity Numbers Mean?

The primary task of engine oil is to keep the engine components lubricated and reduce friction between moving parts. Therefore, the engine oil must be viscous enough to protect the engine at operating temperatures and act as a protective film between two moving parts to prevent metal-to-metal contact. However, it must also not be too viscous to resist flow at cold temperatures when the engine is started. This means when the oil is used at different temperatures, the change in its viscosity must be as minimal as possible.

The Society of Automobile Engineers (SAE) has created a numeric system called J-300 Engine Oil Viscosity Classification (EOVC) that classifies engine oils according to their viscosity characteristics. It includes engine oil grades like‌ 5W30, 10W40, 15W30, etc. These are viscosity grades often referred to as oil weights. The letter "W" stands for Winter, and the number before the letter W shows the oil's flow (viscosity) at zero degrees Fahrenheit (-18 degrees Celsius). If this number is lower, the oil is less viscous at lower temperatures and moves around the engine easily, especially during cold starts. The number after the letter W indicates the viscosity of the oil at 100 degrees Celsius and its resistance to thinning at high temperatures.

Let's understand this with an example. Engine oil with a grade 5W30 is less viscous in cold weather compared to a 10W30 grade oil. However, their viscosity at high temperatures is the same, as indicated by the number 30, which is the same for both engine oils. Similarly, a 5W30 grade engine oil thins out quickly and loses its viscosity at high temperatures compared to an engine oil with a 5W40 grade. The higher number (40) indicates a high viscosity of the engine oil and resistance to thinning at high temperatures.

What Is A Multi-grade Oil?

As mentioned above, the viscosity of engine oil depends on the temperature. A 30W grade oil is optimal for most engines and can perform well at normal operating temperatures. However, at lower temperatures, it becomes way too thick. Similarly, a 10W grade oil performs well in cold conditions but becomes too thin at higher temperatures. 

In order to address this problem, oil companies came up with multi-grade oils that meet both high and low-temperature requirements. To achieve this, oil companies added Viscosity Index Improvers to prevent the engine oil from thinning at higher temperatures. The viscosity index improvers are large-oil soluble polymers that expand and contract with changing temperatures. This helps slow down the rate at which the oil viscosity changes as the temperature increases and allows a 10W grade engine oil to behave like a 30W grade engine oil when hot. This leads to the grading of oil as 10W30, a multi-grade oil that can be used across a wide temperature range.

Back in earlier times, engines used to run on mono-grade oils, also known as straight-grade or single-grade oils, that were suitable either for summers or winters. Single-grade oils were typically used in older vehicles before multi-viscosity oils were introduced. These single-grade oils are denoted by a single number like SAE30 (30W), SAE40 (40W), etc. With the introduction of multi-grade oils, seasonal oil changes were no longer necessary and can be used all year round. Almost all motor oils in the market today are multi-grade oils and can be recognized by typical SAE classes like 5W30, 20W40, etc.

How Does Engine Oil Viscosity Affect Your Engine?

The viscosity of engine oil can be understood by the SEJ-300 chart. It shows the minimum and maximum allowable temperatures for engine oil to be rated for a particular viscosity grade. The W-rating, also known as the winter rating, of oil is determined by its cold-cranking performance. The lower the W-rating (e.g., 0W or 5W), the faster oil flows when the engine is cold and the easier it is to turn the engine while starting. To put it simply, the engine oil grades are viscosity indicators that dictate how an engine oil will perform at different temperatures. 

Engine oils with higher viscosity can lead to poor cold starts, whereas engine oils with lower viscosity can increase metal-to-metal contact, friction, and wear. When the temperature drops, the engine oil thickens, flows slowly, and requires energy to circulate. This makes it tough for you to crank the engine during cold starts. Furthermore, if the oil flows slowly, engine components are vulnerable to wear until the oil warms up and flows throughout the engine for lubrication. The opposite happens when the engine is hot because the oil becomes thin with rising temperature, and a low-viscosity oil may fail to adequately separate metal components, inviting wear and tear.

How To Choose The Best Engine Oil Grade For Your Vehicle?

All vehicle manufacturers today recommend the use of multi-grade engine oil. Usually, the recommended oil grade is mentioned in your owner's manual or on the oil filter cap. Some manufacturers may also suggest different grades of oil for different ambient temperatures. As we discussed earlier, the number on the left of the letter W shows the cold viscosity rating of the oil, and the number to the right shows the hot viscosity rating. According to the engine oil viscosity chart below, the temperature range of a 5W30 grade engine oil is -35 to +35 °C. The chart doesn't show 0W20, but the temperature range for such an engine oil would be -40°C to +15°C.

Therefore, the decision as to which engine oil you should choose depends on the climate you live in. If you live in a very cold area, a low-viscosity grade is a better option. In the extreme winters and snowy conditions of Canada, you can choose lower-grade engine oils like 0W20. These lower-grade engine oils will minimize start-up wear and flow better in the cold. Major engine wear occurs during cold starts, therefore we strongly recommend using an engine block heater in sub-zero temperatures to heat the engine faster and prevent damage. During summers, you can use higher viscosity grade oils like 10W40.

Mineral Oil Vs Synthetic Oil - Which One Should You Choose?

Apart from the oil grades and viscosity ratings, engine oils can be classified as synthetic and mineral oils. Mineral oils are also known as conventional oils that are derived from crude oil. The crude oil refining process separates various types of molecules in the oil by weight. Since the refining process takes weight into account and cannot distinguish between the structure of molecules, mineral oil contains a wide range of molecules of different sizes. This irregularity and non-uniformity at the molecular level reduce the oil's efficiency in lubrication, decreasing heat, and reducing wear. Mineral oils are the most basic type of engine oil used in a wide range of vehicles. Furthermore, they are less resistant to heat and oxidation and have to be changed more frequently.

Synthetic engine oils, on the other hand, are chemically synthesized according to pre-determined performance characteristics. They are engineered with a smooth and uniform molecular composition that helps reduce wear and friction. Compared to mineral oils, synthetic engine oils do not break down or form sludge, leading to cleaner engines and less deposit formation. Synthetic oils are also able to hold their viscosity during changing temperatures more efficiently than mineral oils. They are comparatively expensive but last longer with less frequent changes. Regarding safety, synthetic oils have a higher flash point than mineral oils, making them less flammable and preferable in high-temperature applications.

The advantages of choosing synthetic engine oil over mineral oils are numerous. Even the American Automobile Association (AAA) states that it's worth paying a little extra for synthetic oil over conventional oil. Additionally, the superior film strength, oxidation resistance, and high and low-temperature performance provide unsurpassed protection from wear and tear, offering an extended engine life. A blend of mineral oil and synthetic oil is known as semi-synthetic oil or synthetic blend. They are a good compromise between mineral oils and synthetic oils and they outperform mineral oils in terms of performance and oil change intervals.

Oil Filter - Your First Defence Against Engine Wear

An oil filter is as important as an engine oil for your vehicle and acts as the first line of defense against engine wear. Automotive experts agree that dirt and engine oil contaminants are the number one cause of engine damage. Several analyses show that over 40 percent of all engine bearing issues are caused by dirt or contaminated oil. As the engine components wear, the metal particles get carried by the oil into precision clearances between bearings and other parts, where they grind and score the surfaces. As this wear cycle continues, the contaminants damage the engine until it fails. 

Therefore, proper filtration is the key to keeping your engine oil clean and the engine safe from potential issues. As important as it is to choose the correct engine oil for your vehicle, it is also important to know the difference between a cheap and good quality oil filter. There are several factors like dirt holding capacity, micron size, filter element, etc. that determine the filtration efficiency of an oil filter. Neglecting a little research before purchasing an oil filter can be detrimental to the vehicle’s performance. This detailed oil filter buying guide covers the best oil filters from top brands ‌you can choose from and will help you make an informed decision.


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John Framigllia
Technical Writer
Our technical writer is known for simplifying automotive parts and services. Intuitive with various vehicles and manufacturers, he knows how to simplify complicated problems.