5W-20 vs. 5W-30 Oil: What’s The Difference?

What's the real difference between 5W-20 and 5W-30 oil and can you mix them? Here's the ultimate guide for a quick overview.

You may have read a magazine or online article comparing 5W-20 vs. 5W-30 oil. These often suggest one is better than the other. Both are modern multi-grade motor oils, and they’re broadly quite like each other.

However, each oil is more efficient in specific circumstances and environments. This guide will compare 5W-20 vs. 5W-30 and explain the difference between them. Most importantly, it will delve into why you should pick one or the other.

You can then use this knowledge when you next change your oil. If you take your car for its regular service, the mechanic should know what type of engine oil they need to use.

However, you could always ask them to use your chosen grade. Even if they refuse, they’ll explain why that particular type does or doesn’t work for your vehicle.

Tailored advice for your specific car, model, and climate (Alaska, California, Florida, and New York all have very different weather patterns, average humidities, and temperature ranges!) is much more beneficial than a generic online article.

Here’s the bottom line up front: read your owner’s manual.

Turn to the page that talks about engine oil. Here, you’ll find exactly which type you should be using. It’s country-specific, too. Any of these oils should be fine. Don’t use any multi-grade or monograde engine oils you don’t find here.

With that said, let’s begin looking at 5W-20 vs. 5W-30. What is the difference, and what should you do with that information?

Related:0W-20 vs 5W-20 Oil: What’s The Difference?

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Multi-Grade vs. Monograde Motor Oil

Multi Grade Engine Oil 15W 40

Cars became more popular in the mid-1900s, particularly in the United States. It was far ahead of the rest of the world, most likely due to increased demand and longer journeys.

Most cars would have needed two oil changes throughout the year: one at the start of summer and one before winter. The oil became too thick in the colder months for the pump to push it efficiently through the engine. This made starting the car nigh-on impossible.

Instead, mechanics would put thinner oil in the car in winter. This would work well until the weather started warming up again. At this point, the oil’s too thin to cope with the heat. Thus, a technician would again drain it and refill the sump with the thicker oil.

And thus, this process continued year after year. The invention of multi-grade oils changed all that.

What Is Viscosity?

The topic of viscosity and flow rate will always come up when talking about motor oils. Although it can sound confusing at first, you’ll quickly pick the idea up.

The best way to think about viscosity is how “thick” a liquid is. The “thicker,” the “more viscous.” For example, runny honey is less viscous than set honey, and custard is more viscous than water.

In more scientific terms, viscosity is a liquid’s internal resistance to its own flow. That is, how much it slows itself down due to its molecules’ frictional collisions.

What Are Multi-Grade Oils?

0W 20 vs 5W 20 Oil

Multi-grade oils are, like monograde oils (and diesel, gasoline, kerosene, etc.), distilled from raw crude oil in a refinery. Distilleries add molecules known as VI Improvers, along with many other additives, to improve engine health.

VI Improvers – Viscosity Index Improvers – are a little like a slinky. When they’re cold, they contract (like all molecules) like a slinky in its resting position. However, under heat, they expand (like stretching out a slinky). This increases the viscosity by increasing resistance while the liquid flows.

Because of this, you can use multi-grade oils all year round. They’ll be thin during cold weather, allowing the oil pump to work with minimal effort. This helps preserve your car battery.

As the engine warms to around 100 degrees C (212 degrees F), it becomes more viscous (thicker). As a result, it better coats and lubricates the engine’s metal parts.

What Are SAE Grades In Multi-Grade Oils?

Multigrade vs Monograde Engine Oil Viscosity

The Society of Automotive Engineers, now known as SAE International, sets the standards for many international automotive practices. Although it was and still is based in the United States, it operates globally.

The SAE regulates motor oil classification with SAE viscosity grades, also casually known as SAE numbers or SAE ratings.

For monograde oils, two standardized laboratory tests are carried out on the oil. These measure kinematic viscosity. One is a viscosity test at 100 degrees C, and the other is a high shear rate viscosity test (at 150 degrees C). The results of this test give the oil a rating. The higher the rating, the more viscous the flow is.

The results of these two tests are given as a relative rating, known as the SAE viscosity grade. They’re generally between SAE 20 and SAE 60. Modern cars sometimes reach SAE 16. This new category was introduced in 2013 to account for ever-more efficient engines.

When it comes to multi-grade motor oils, though, they do another two tests. These establish the dynamic viscosity and are done at ever-decreasing cold temperatures. The better the oil fares in the cold (reaching -35 and -40 degrees, respectively), the lower its rating.

This is the W grade you notice in multi-grade oils. It can be between 0W and 20W, although these days, you’d rarely see oil above 10W.

Explaining 5W-20 vs. 5W-30 Oil

motor oil

So, with all that established, it’ll be much easier to talk about 5W-20 vs. 5W-30 oil.

5W-20 has a rating of 5W in the cold and a grade of 20 under normal operating temperatures. This means it’s thin when you’re trying to cold-start your car but also relatively thin when the engine’s running.

5W-30 is also rated 5W in winter conditions, with an SAE grade of 30 when the engine’s warmed up, and you’re driving. This oil behaves the same as 5W-20 in the cold. However, it becomes comparatively thicker when flowing under standard running temperatures.

For a long time, 5W-30 was the most common motor oil. It was widely available and used worldwide. Manufacturers would build their cars with engines to match this multi-grade rating.

Nowadays, though, it’s vital for our transportation vehicles to be environmentally-friendly. In cars, this means smaller, more efficient engines with turbochargers and superchargers. Sorry, V8 fans – V8s may well survive, but they won’t be commonplace before long.

The smaller and more efficient an engine is, the less viscous the oil needs to be. As such, most modern cars are constructed to take multi-grade oils like 0W-20 or 5W20. 0W-16 and 5W-16 aren’t that unusual anymore, although they can still be quite expensive.

5W-20 is likely to slowly catch up and overtake 5W-30 in market popularity.

5W-20 vs. 5W-30 – Which Is Better For Your Car?

5W 20 vs 5W 30 Oil

As mentioned, both 5W-20 and 5W-30 perform the same when cold-starting. 5W-30, though, is slightly thicker than 5W-20 when the engine’s up to temperature and you’re driving along.

So, which is better?

That’s a trick question. There’s no one-size-fits-all answer. 5W-20 isn’t necessarily better because it’s less viscous; neither is 5W-30 necessarily more beneficial because it’s thicker.

Here’s the thing. Despite what countless online blogs, forums, and social media groups might tell you, manufacturers know what they’re doing when designing and building engines. With massive Research & Development departments, highly-qualified engineers, and millions of dollars at their disposal, developing a basic vehicle still takes five years.

When they design, build, test, and adjust new engines, manufacturers carefully create them to work with certain types of engine oil. They might modify this depending on the target market’s climate, but it’ll always be within specifications.

Your owner’s manual will list the motor oil you should use. Find the glossary at the back and turn to the page number listed next to “Motor Oil,” “Engine Oil,” or “Servicing Details”– something like that. On one of these pages, you’ll find the listed types of multi-grade oil you should use, depending on the specific engine.

Sometimes, you’ll have more than one option. That’s great, and in that case, either should be fine. But you should never use oil you don’t find in the owner’s manual.

What If My Owner’s Manual Says I Can Use Either 5W-20 Or 5W-30?

Service Manual

Sometimes, your owner’s manual will give multiple options. These could include 5W-20 or 5W-30. In this case, it’s up to you.

The cold-start “W” grade has the most significant impact in most instances. If you live anywhere where temperatures can reach 0 degrees C or below, you’ll want to take the lowest possible W number.

However, when comparing 5W-20 vs. 5W-30, both have the same winter rating. It’s the operating (kinematic) viscosity that’s different. In this case, it probably won’t make any noticeable difference.

Overall, use 5W-30 if you often put your engine under significant stress for sustained periods. Examples include fast highway driving, climbing up hills, or hard acceleration. 5W-20 might be more appropriate for smoother drivers.

The reason for this comes down to engine heat. 5W-30 is more suited to handling heavier loads and higher temperatures. 5W-20 is marginally better for lower temperatures. The trade-off is that 5W-30 will use a tiny fraction more gas.

Since 5W-30 is more widely available, it’s often cheaper, so it might make sense to go down that route. It also offers your engine slightly more high-temperature protection.

If your owner’s manual says you can use either, you should be fine.

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Benjamin Kitchen

Ben is an automotive author from England. With experience in a fast-fit garage, he's an IMI-qualified light vehicle technician. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa – it may have a tiny engine, but in eight years it's never once let him down!

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