Can I Mix Engine Oil 5W20 And 5W30 – The Definitive Answer For DIY

ByRobert Lozano

In a pinch, yes, you can mix 5W-20 and 5W-30 engine oil. It’s a far better option than running your engine with dangerously low oil. However, this should only be a temporary solution to top off your oil level, not a long-term strategy or a full oil change substitute.

You’re in the garage, halfway through a weekend project, or maybe you’re on a long road trip when the dreaded oil light flickers. You check the dipstick and confirm you’re a quart low. The only bottle of oil you have on hand is 5W-30, but your engine calls for 5W-20. It’s a classic DIY dilemma that can leave you wondering if you’re about to make a costly mistake.

We get it. The last thing you want to do is harm your engine. That’s why we’re here to give you the straight, no-nonsense answer on the question: can i mix engine oil 5w20 and 5w30? We promise to cut through the noise and give you the practical, real-world advice you need to protect your vehicle.

In this complete guide, we’ll break down what those numbers on the bottle actually mean, explore the exact scenarios where mixing is acceptable, detail the potential risks, and provide you with the best practices to keep your engine running smoothly for years to come.

Decoding the Numbers: What Do 5W-20 and 5W-30 Really Mean?

Before we can talk about mixing, we need to understand what we’re working with. Those numbers on your oil bottle aren’t just for decoration; they represent the oil’s viscosity, which is its resistance to flow. The Society of Automotive Engineers (SAE) sets these grades.

The “5W” – Cold Start Performance

The first part of the grade, the “5W,” tells you how the oil behaves in cold temperatures. The “W” stands for winter.

A lower number here means the oil is thinner and flows more easily when the engine is cold. This is crucial because most engine wear happens during startup, before the oil has had a chance to circulate fully.

Both 5W-20 and 5W-30 share the same cold-weather viscosity rating of “5W.” This is the most important takeaway: during a cold start, both oils are designed to perform identically, providing the same level of protection when your engine is most vulnerable.

The “20” vs. “30” – Operating Temperature Viscosity

The second number—the “20” or “30”—indicates the oil’s viscosity once the engine has reached its full operating temperature (around 212°F or 100°C).

Here’s the difference:

  • 5W-20 is thinner at operating temperature. Modern engines are often designed with tighter tolerances and specify this lighter oil to improve fuel efficiency and circulation.
  • 5W-30 is thicker at operating temperature. This provides a slightly more robust film of protection under higher heat and load, which is why it’s often specified for high-performance or older engines.

The Big Question: Can I Mix Engine Oil 5W20 and 5W30?

Now for the main event. Based on what we just learned, you can probably guess the answer. Since both oils have the same cold-start properties and are from the same family of multi-grade oils, mixing them isn’t catastrophic.

The short answer is: Yes, you can mix 5W-20 and 5W-30 in an emergency top-off situation. It is significantly better to have a mix of slightly different viscosities than to run the engine low on oil, which can cause severe and immediate damage.

When It’s Generally Okay (The “Top-Off” Scenario)

The most common and acceptable reason to mix these oils is when you need to add a quart or less to an engine that is low.

When you add a quart of 5W-30 to an engine filled with 5W-20 (or vice versa), you are slightly altering the overall viscosity. The resulting mixture will be somewhere between a 20 and 30 weight oil at operating temperature—perhaps a “5W-26” if you want to get technical. For a temporary period, this minor change is unlikely to cause any harm.

This is a core part of any practical can i mix engine oil 5w20 and 5w30 care guide: prioritize having enough oil over having the perfect oil for a short time.

When You Should Absolutely Avoid It

While a top-off is okay, there are situations where you should be much more cautious. Deliberately mixing for a full oil change is not recommended.

  • Engines Under Warranty: If your vehicle is still under its manufacturer’s warranty, stick to the exact oil specified in your owner’s manual. Using a different blend could potentially give the dealer a reason to deny a warranty claim related to the engine.
  • High-Performance or Turbocharged Engines: These engines operate under extreme heat and pressure. They are precision-engineered for a specific oil viscosity to ensure components like the turbocharger are properly lubricated and cooled. Don’t experiment here.
  • Variable Valve Timing (VVT) Systems: Many modern engines use VVT systems that are operated by oil pressure. These systems can be highly sensitive to viscosity. Using an oil that is too thick or too thin can affect their performance and potentially trigger a check engine light.
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The Real-World Consequences: Short-Term vs. Long-Term Effects

Understanding the common problems with can i mix engine oil 5w20 and 5w30 is about looking at the immediate and future impact. One is minor, the other can be significant.

Short-Term Impact: What Happens Right Away?

If you just topped off your 5W-20 engine with a quart of 5W-30, you will likely notice… nothing at all. The change in overall viscosity is too small to have a perceivable effect on engine sound or performance for the average driver.

Theoretically, the slightly thicker oil might cause a minuscule drop in fuel economy (less than 1%), but you’d be hard-pressed to measure it without specialized equipment. The immediate benefit—preventing engine damage from oil starvation—far outweighs this tiny drawback.

Long-Term Risks of Consistently Using the Wrong Mix

This is where the real danger lies. If you continuously use a 50/50 mix or run 5W-30 in an engine designed for 5W-20 (or vice versa), you’re inviting trouble down the road.

  • Increased Wear: An engine designed for the thinner 5W-20 oil might struggle to pump the thicker 5W-30 through its narrow passages, especially on VVT systems. This can lead to delayed lubrication and increased wear over tens of thousands of miles.
  • Reduced Fuel Economy: Consistently using a thicker oil than specified will create more internal drag, forcing the engine to work harder and reducing your miles per gallon.
  • Voided Warranty: As mentioned, this is one of the quickest ways to run into issues with a manufacturer’s warranty claim on your powertrain.

Best Practices and a Practical Guide for Mixing Oils

Following a clear can i mix engine oil 5w20 and 5w30 guide ensures you make the right call every time. These are the rules we follow in our own shop and on the trail.

The Golden Rule: Your Owner’s Manual is King

Before you do anything, check your owner’s manual. It is the ultimate source of truth for your vehicle. It will specify the required oil viscosity, API certification (like API SP), and any other special requirements. Always default to the manufacturer’s recommendation for full oil changes.

A Step-by-Step Emergency Top-Off Guide

If you find yourself in a bind, here’s how to can i mix engine oil 5w20 and 5w30 safely:

  1. Confirm the Need: Park on a level surface, wait a few minutes for the oil to settle, and check the dipstick. If the level is at or below the “ADD” mark, you need to add oil.
  2. Assess Your Options: The best option is always the exact oil specified for your car. If that’s not available, a different viscosity of the same type (e.g., full synthetic 5W-30 for a full synthetic 5W-20 engine) is your next best choice.
  3. Add Sparingly: Use a funnel and add about half a quart at a time. Wait a minute and re-check the dipstick. Add more as needed to bring the level to the “FULL” mark, but do not overfill.
  4. Plan Your Next Move: The mix will get you home safely, but you should plan to get a proper oil and filter change at your earliest convenience to restore the manufacturer’s specified viscosity.

Eco-Friendly and Sustainable Considerations

It might seem like a good idea to mix leftover partial bottles of 5W-20 and 5W-30 to reduce waste. While the intention is good, this isn’t the best approach for a sustainable can i mix engine oil 5w20 and 5w30 strategy.

Your engine’s health and efficiency are paramount. The most eco-friendly can i mix engine oil 5w20 and 5w30 practice is to use the correct oil to maximize fuel economy and engine life. Take all your used and leftover oil to an auto parts store or a local recycling center for proper disposal.

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Mixing Different Brands and Types: A Word of Caution

The viscosity question is just one piece of the puzzle. What about mixing brands or oil types?

Mixing Brands (e.g., Mobil 1 and Valvoline)

This is generally not a problem. As long as both oils meet the API service rating specified in your owner’s manual (e.g., API SP or ILSAC GF-6), they are formulated to be compatible. You can safely mix a quart of Valvoline 5W-20 with a crankcase full of Mobil 1 5W-20.

Mixing Synthetic, Synthetic Blend, and Conventional

Here’s where it gets tricky. All modern oils are miscible, meaning they will mix without separating. However, mixing types dilutes the quality of the better oil.

If you add a quart of conventional oil to your full synthetic fill, you’ve just compromised the superior performance, temperature resistance, and longevity of the synthetic oil. You’re effectively downgrading your oil change and should shorten your change interval accordingly.

Frequently Asked Questions About Mixing 5W-20 and 5W-30

What happens if I accidentally put 5W-30 in my 5W-20 engine for a full oil change?

Don’t panic. For a single oil change interval, this is unlikely to cause any damage, especially in a warmer climate or on an older vehicle. You might see a very slight decrease in fuel economy. However, you should switch back to the recommended 5W-20 at your next oil change.

Can I mix 5W-20 and 5W-30 in an older, high-mileage car?

In high-mileage cars, some mechanics actually recommend switching to a slightly thicker oil like 5W-30 (from 5W-20) to help reduce oil consumption and quiet down a noisy engine. In this context, mixing to top off is perfectly fine. Just be consistent and don’t switch back and forth frequently.

Will mixing these oils cause my engine to sludge up?

No. Mixing 5W-20 and 5W-30 of the same type (e.g., both are API-certified synthetics) will not cause sludge. Sludge is typically caused by oil breakdown from extreme heat, moisture contamination, or going way too long between oil changes—not from a minor viscosity blend.

Is it better to run a quart low or top off with the wrong weight?

Always top off. Running an engine a quart or more low on oil is one of the most damaging things you can do. The risk of oil starvation, overheating, and catastrophic failure is very real. Topping off with a slightly different viscosity is a minor, temporary compromise.

Your Final Checklist for Smart Oil Management

So, can you mix 5W-20 and 5W-30? The answer is a qualified yes. It’s a tool for your emergency toolkit, not a regular maintenance practice.

Remember these key takeaways:

  • For Emergencies Only: Mixing is for topping off a low engine to get you home or to the auto parts store. It’s not for a full oil change.
  • Your Manual is Law: Always use the oil viscosity recommended by your vehicle’s manufacturer for optimal performance and engine life.
  • Better Than Low: Any clean oil of a similar grade is better than not enough oil. Don’t hesitate to top off if you’re low.
  • Get Back to Basics: After mixing, make a plan to perform a full oil and filter change with the correct grade of oil as soon as you can.

Keeping on top of your vehicle’s maintenance is one of the most rewarding parts of being a car owner or DIYer. Making smart, informed decisions is what separates a good mechanic from a great one.

Keep those wrenches turning and your engine happy. Drive safe!

Robert Lozano
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