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Oil Viscosity: What It Is + How It’s Measured + 7 FAQs

November 29, 2021

Viscosity is one of the most critical properties of engine oil. 

It dictates how oil flows through and coats moving engine parts to keep them from grinding against each other. It also affects how well engine heat is transferred.

So, what exactly is oil viscosity? 

We’ll discuss how oil viscosity is defined, including the difference between dynamic viscosity and kinematic viscosity. And if you’re curious about the viscosity index, we’ve got that covered too, plus several FAQs to help further clarify engine oil viscosity.

This Article Contains:

Let’s get cranking.

What Is Oil Viscosity?

Viscosity describes how resistant a fluid is to flow. It indicates how thin or thick fluid is. 

Here’s an easy way to think of viscosity: 

Oil thins out as it gets warmer, so engine oil viscosity refers to how well it pours at a particular temperature.  

Engine lubricant viscosity is usually defined through its kinematic viscosity and dynamic viscosity (absolute viscosity). Another important viscosity indicator is the viscosity index. 

Let’s take a look:

A. Kinematic Viscosity

Kinematic viscosity is fluid resistance to flow and shear due to gravity. 

If you pour water into a container and pour honey into another, you’ll notice that water flows faster. This is because water has a lower kinematic viscosity than honey. 

The high temperature viscosity grade of oils is determined by its kinematic viscosity (typically tested to ASTM D445). And this value is usually reported at either 40°C (100°F) or 100°C (212°F).

For motor oils, kinematic viscosity is usually measured at 100°C as this is the temperature that the SAE engine oil classification refers to. 

B. Dynamic Viscosity (Absolute Viscosity)

Dynamic viscosity (or absolute viscosity) is slightly different from kinematic viscosity. 

Let’s say you used a straw to stir first water, then honey. 

You’d need more effort to stir the honey because it has a higher viscosity than water. Dynamic viscosity refers to the amount of energy required to move an object through a fluid. 

For motor lubricant, dynamic viscosity determines the oil’s cold temperature viscosity grade (the “W” rating). It’s measured via the Cold Cranking Simulator test, which simulates engine startup at progressively lower temperature settings. 

C. Oil Viscosity Index

Viscosity Index (VI) is a unitless number representing how much the kinematic viscosity of a lubricant changes with temperature. 

It’s obtained by comparing a test oil’s kinematic viscosity at 40°C with the kinematic viscosity of two reference oils. One of the reference oils has a VI of 0, and the other has a VI of 100. All three oils have the same viscosity at 100ºC.

If the test oil’s viscosity doesn’t change much between 40°C to 100ºC, it will have a high viscosity index — meaning its viscosity is relatively stable with different temperatures. Many refined conventional and synthetic lubricant formulations have a viscosity index exceeding 100.

Next, let’s explore some FAQs related to oil viscosity.

7 FAQs About Engine Oil Viscosity

Here are the answers to some common oil viscosity questions:

1. Who Designed Oil Viscosity Grades?

Oil viscosity grades for engine and transmission oils (SAE J300) were developed by the Society of Automotive Engineers (SAE).

2. What Are Multigrade Oils?

Before multi grade oils were developed, most vehicles used one viscosity grade oil in the winter and another for summer. 

As motor oil technology evolved, additives like Viscosity Index Improvers (VII) allowed for multigrade oils. These oils have two viscosity grades so that the same motor oil grade can be used all year.

3. What Do Multigrade Oil Numbers Mean?

SAE oil viscosity grades are in an “XW-XX” format, where “W” stands for Winter.

The number before “W” is the low temperature oil viscosity. It’s measured at -17.8°C (0°F) and simulates the vehicle startup conditions in winter. The lower this number is, the thinner the oil in lower temperature settings. 

So, 0W-20 is pretty smooth-flowing, low viscosity oil in cold startups.

The number after the “W” is the high temperature oil viscosity. Measured at 100°C (212°F), it represents the oil flow at an engine’s operating temperature. The higher the number, the more resistant the oil to thinning at higher temperatures. 

Meaning, 10W-40 would be a great high viscosity oil for heavy-load, high temperature applications.

Note: Gear oils have a similar SAE grading format to engine lubricant, but their classifications aren’t related. Engine and gear oils with the same viscosity will have markedly different SAE viscosity grade designations.

4. What Happens When Engine Oil Viscosity Is Too Thin?

Oils with lower viscosity are good for cold startups, but when a thin oil is too thin for your engine, here’s what can happen:

5. What Happens When Engine Oil Viscosity Is Too Thick?

A higher viscosity oil is ideal for heavy-loads and warm climates, but if it’s too thick, it can hurt your engine in these ways:

6. What Are Popular Engine Oil Viscosity Grades?

The most commonly used motor oil viscosity grades are 5W-30 and 5W-20, with 0W-20 gaining popularity in recent times. 

These thinner multi grade oils have gained precedence over previously preferred thicker oil grades like 20W-50 or 10W-30 due to the narrower oil pathways in smaller, modern engines. 

Tighter gaps in engine parts require a lower viscosity oil, with the added benefit of better fuel economy from a motor oil that flows quickly. 

7. Does Motor Oil Type Affect Oil Viscosity?

For the most part, no. 

The same motor oil viscosity can exist in conventional oil, synthetic blend, or full synthetic oil types. They’ll contain additives like viscosity index improvers, friction modifiers, anti-wear additives, and more to provide efficient engine protection and performance.

However, very low viscosity winter-grade oils like 0W-20 or 0W-30 only come as a synthetic blend or full synthetic oil.

Why?

Conventional oil is only refined from crude oil and contains many impurities. Synthetic base oil is chemically engineered to create uniformly-shaped molecules with fewer impurities. This allows synthetic oil to flow at a much lower temperature. 

Closing Thoughts

Knowing how different motor oil viscosities can affect your engine’s performance, longevity, and fuel consumption is a significant part of car care, on top of how often an oil change is needed. 

The best place to find the right viscosity is your vehicle owner’s manual. The manual may recommend different oil grades depending on where the car is driven, as the climate is an important selection factor. 

And if you need help with an oil change, you can always get hold of RepairSmith!

RepairSmith is a mobile vehicle repair and maintenance solution that offers easy online booking and is available 7 days a week. Not only can we help with an oil change, but we can provide most services your vehicle might need directly on-site.

Simply contact us, and our ASE-certified mechanics will pop by to lend you a hand, right in your driveway!