Can 5W40 Be Used Instead Of 5W20 – A Mechanic’S Guide To Engine

ByRobert Lozano

Using 5W-40 oil in an engine designed for 5W-20 is generally not recommended and can cause problems. The thicker 5W-40 oil can struggle to lubricate tight clearances in modern engines, potentially leading to reduced fuel economy, increased engine strain, and issues with systems like Variable Valve Timing (VVT).

Introduction

You’re standing in the auto parts aisle, staring at a wall of motor oil bottles. You know your owner’s manual calls for 5W-20, but the 5W-40 is on sale, or maybe it’s the only option left on the shelf. The thought crosses your mind: “They’re both ‘5W’—how different can they be?”

This is a common crossroads for every DIYer, and making the wrong choice can have real consequences for your engine. This guide will cut through the confusion and give you the straight-up, mechanic-approved answer to the question, can 5w40 be used instead of 5w20? We promise to deliver clear, practical advice based on real-world experience.

We’ll break down what those numbers really mean, explore the risks of using the wrong oil, identify the rare exceptions to the rule, and give you the confidence to choose the right lubricant every single time. Let’s get your engine the protection it deserves.

Decoding the Numbers: What Do 5W-20 and 5W-40 Actually Mean?

Before we can talk about swapping oils, we need to understand what we’re looking at. Those numbers on the bottle aren’t just for decoration; they represent the oil’s viscosity, which is just a fancy word for its thickness or resistance to flow.

Modern motor oils are “multi-grade,” meaning they have two viscosity ratings. Let’s use 5W-20 as our example:

  • The “5W” Part: The number followed by the “W” stands for “Winter.” This rating indicates how easily the oil flows at cold temperatures (like during a cold start). A lower number means the oil is thinner and flows better in the cold, providing crucial lubrication to your engine’s internal parts faster.
  • The “20” Part: This second number represents the oil’s viscosity when the engine is at its normal operating temperature (around 212°F or 100°C).

The Key Difference: Thickness at Operating Temperature

Both 5W-20 and 5W-40 behave similarly in the cold—they both have a “5W” rating. The critical difference is how they perform once your engine is warmed up.

A “40” grade oil is significantly thicker and more viscous at operating temperature than a “20” grade oil. Think of it like the difference between maple syrup (5W-20) and honey (5W-40). While both pour, the honey is much thicker and flows more slowly.

So, Can 5W40 Be Used Instead of 5W20? The Short and Long Answer

Here’s the straightforward answer you came for: No, you should not use 5W-40 oil in an engine that specifically requires 5W-20.

Modern engines are not like the engines of 30 years ago. They are built with incredibly tight internal clearances—the tiny spaces between moving parts like bearings, pistons, and camshafts. Engineers design these engines to work with the specific flow characteristics of a thinner oil like 5W-20.

Using a thicker oil like 5W-40 is like trying to force honey through a drinking straw. It disrupts the delicate balance the engineers designed, and this is where you run into trouble.

The Real-World Risks: Common Problems with Using Thicker Oil

Ignoring your manufacturer’s recommendation isn’t just a minor misstep; it can lead to tangible issues. This isn’t about scare tactics; it’s about understanding the mechanics. Here are the common problems with can 5w40 be used instead of 5w20 that we see in the shop.

1. Starving Critical Components of Oil

On a cold start, your engine needs oil to circulate immediately. While both oils are 5W, the thicker 5W-40 will still be slightly slower to pump through the engine’s narrow passages. More importantly, once warm, the thicker oil can struggle to penetrate those tight tolerances, leading to increased wear over time.

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2. Impaired VVT System Performance

Many modern engines use a Variable Valve Timing (VVT) system to improve performance and fuel efficiency. These systems rely on oil pressure, delivered through tiny solenoids and passageways, to adjust the engine’s timing.

A thicker oil like 5W-40 can flow too slowly or fail to build pressure correctly in these sensitive systems. This can lead to a check engine light, rough idling, poor performance, and failed emissions tests.

3. Reduced Fuel Economy

Your engine’s oil pump has to work harder to push thicker oil throughout the system. This extra work creates more internal drag, or “pumping losses.” The engine has to burn more fuel to overcome this resistance, resulting in a noticeable drop in your miles per gallon (MPG).

4. Increased Engine Strain and Heat

The increased effort required to pump thicker oil puts more strain on the oil pump and other components. This extra work generates more heat, which can accelerate oil breakdown and contribute to long-term wear on engine parts.

Are There Any Exceptions? When Thicker Oil Might Be Okay

While the golden rule is “use what the manual says,” the world of mechanics is full of nuances. There are a few very specific scenarios where running a slightly thicker oil might be considered, but these are exceptions, not the rule. This is not a general “how to can 5w40 be used instead of 5w20” for every car.

High-Mileage or Worn Engines

In an older engine with significant mileage (think 150,000+ miles), internal clearances can widen due to wear. In this case, a slightly thicker oil like 5W-30 (not necessarily 5W-40) might help maintain oil pressure and reduce oil consumption or minor leaks. Consult a trusted mechanic before making this switch.

Dedicated Track Use or Extreme Racing

If you’ve heavily modified your car for racing, you’re operating far outside the manufacturer’s original design. Under the extreme heat and stress of track conditions, a thicker oil like 5W-40 can provide a more robust lubricating film, protecting bearings from high loads. This choice should be made based on data and expert advice from engine builders, not a guess.

Extreme Climate Conditions

If you live in and operate your vehicle exclusively in a scorching desert climate where ambient temperatures are consistently above 100°F (38°C), some manufacturers may list a thicker oil as an acceptable alternative in the owner’s manual. Always verify this in your specific manual.

A Practical Guide to Making the Right Choice: Best Practices for Your Engine

Navigating your engine’s needs doesn’t have to be complicated. This can 5w40 be used instead of 5w20 guide boils down to a few simple, reliable steps. Following these best practices will ensure your engine stays healthy for the long haul.

  1. Trust Your Owner’s Manual: This is your vehicle’s bible. The engineers who designed your engine spent thousands of hours determining the optimal oil viscosity. Always start and end here.
  2. Check the Oil Cap: Most manufacturers print the recommended oil weight directly on the oil filler cap. It’s a quick and easy reminder right in the engine bay.
  3. Consider Synthetic Oil: If your manual allows it, upgrading to a full synthetic 5W-20 offers superior protection, better temperature stability, and longer life compared to conventional oil.
  4. When in Doubt, Stick with 5W-20: If you’re ever unsure, defaulting to the manufacturer’s specified 5W-20 is the safest, most reliable choice for your engine’s health and longevity.
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Eco-Friendly and Sustainable Oil Choices: Beyond Viscosity

Being a responsible car owner also means thinking about the environmental impact of our maintenance. When considering oil, a sustainable can 5w40 be used instead of 5w20 approach focuses on efficiency and proper disposal.

Using the correct oil (5W-20 in this case) is the most eco-friendly can 5w40 be used instead of 5w20 choice because it maximizes your engine’s efficiency. Better efficiency means you burn less fuel, which reduces your overall emissions.

Furthermore, always recycle your used motor oil and filters. Most auto parts stores and local municipal waste facilities accept used oil for free. Never pour old oil down the drain or on the ground—it’s highly toxic and harmful to the environment.

Frequently Asked Questions About Using 5W-40 Instead of 5W-20

What if I accidentally put 5W-40 in my 5W-20 car?

If it’s just for one oil change, it’s unlikely to cause immediate, catastrophic damage. However, you might notice slightly lower fuel economy or a sluggish feeling. We recommend changing it back to the correct 5W-20 at your earliest convenience to avoid potential long-term issues with VVT systems or wear.

Will 5W-40 cause my check engine light to come on?

Yes, it absolutely can. The most common cause would be the thicker oil interfering with the proper operation of the Variable Valve Timing (VVT) system, which can trigger a fault code and illuminate the check engine light.

Is 5W-40 better for off-roading than 5W-20?

Not necessarily. Unless you’re operating in extremely high-heat, low-speed crawling conditions for extended periods, the factory-recommended 5W-20 is still the best choice. It ensures proper lubrication to all parts of the engine, which is just as critical on the trail as it is on the highway.

Can I mix 5W-20 and 5W-40 oil?

While oils are compatible and won’t turn to sludge if mixed, it’s poor practice. You’ll end up with an oil of an unknown viscosity that doesn’t meet the specifications for either grade. If you need to top off in an emergency, a small amount is better than running low, but you should change the oil to the correct grade as soon as possible.

Your Final Takeaway: Stick to the Spec

At the end of the day, the answer is clear. While the allure of a sale or the convenience of grabbing what’s available is strong, the science and engineering behind your engine are stronger.

For the health, efficiency, and longevity of your vehicle, stick with the 5W-20 oil your manufacturer recommends. It’s the simplest and most effective way to ensure your engine performs as it was designed to, keeping you on the road, on the trail, and out of the repair shop.

Grab your funnel, the right oil filter, and a torque wrench for that drain plug, and give your engine the care it deserves. Happy wrenching, and drive safe!

Robert Lozano
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