Mix 5W30 And 5W40 – Your Complete Guide To Safe Topping-Off And Oil

ByRobert Lozano

In a pinch, you can safely mix 5W-30 and 5W-40 motor oil, especially for a small top-off. Since they share the same cold viscosity (“5W”), they are compatible. However, this should be a temporary solution, as the resulting oil blend will have a different viscosity than what your manufacturer recommends, and it’s always best to use the specified grade for a full oil change.

You’re standing in your garage, dipstick in hand, and notice you’re a quart low. You scan your shelf and find a fresh bottle of 5W-40 synthetic, but your engine calls for 5W-30. The auto parts store is closed, and you have to drive to work in the morning. This is a classic DIY dilemma that sends many drivers down a rabbit hole of confusing forum debates.

The anxiety is real. Will mixing these oils cause engine sludge? Will it void your warranty? Or is it a perfectly acceptable solution in a bind? We promise to cut through the noise and give you a clear, expert-backed answer based on real-world mechanical experience.

In this complete mix 5w30 and 5w40 guide, we’ll break down what those numbers on the bottle actually mean, explain the science behind mixing oils, and provide best practices for when it’s okay and when you should absolutely avoid it. Let’s get your engine topped off safely and get you back on the road with confidence.

Decoding the Numbers: What Do 5W-30 and 5W-40 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, or its resistance to flow, at different temperatures. Understanding this is the first step in our mix 5w30 and 5w40 care guide.

The Society of Automotive Engineers (SAE) created this grading system to standardize motor oil performance across all brands.

The “W” and the First Number: Cold Weather Performance

The first part of the grade, “5W,” is often the most misunderstood. The “W” stands for Winter, not “Weight.”

This number indicates how easily the oil flows at cold temperatures, like during a frosty morning startup. A lower number means the oil is thinner and flows better when cold.

In our case, both 5W-30 and 5W-40 have the exact same cold-flow properties. This is the most important reason why mixing them isn’t catastrophic. They both behave identically on a cold start, ensuring critical engine parts get lubricated quickly.

The Second Number: High-Temperature Performance

The second number (the “30” or “40”) tells you about the oil’s viscosity at the engine’s normal operating temperature (measured at 100°C or 212°F).

  • 5W-30 is thinner at operating temperature. This allows for slightly better fuel efficiency because there’s less internal drag.
  • 5W-40 is thicker at operating temperature. This provides a more robust, protective film over engine components under high heat and heavy loads, often favored in European or high-performance vehicles.

So, the only real difference between these two oils is how thick they are once your engine is fully warmed up.

The Big Question: Is It Safe to Mix 5W-30 and 5W-40?

Now for the main event. The short answer is: yes, it is generally safe to mix 5W-30 and 5W-40, especially in a top-off situation. Running your engine a quart low on oil is far more dangerous than running it with a slightly altered viscosity.

Modern motor oils, as long as they meet the same API (American Petroleum Institute) service standard (like API SP or SN), are designed to be compatible. Let’s explore the specific scenarios.

When It’s Generally Okay to Mix

Think of mixing as a temporary fix, not a long-term strategy. Here are the most common situations where it’s perfectly acceptable:

  1. Emergency Top-Off: This is the number one reason. If you’re low on oil and the only thing available is a different grade, adding a quart of 5W-40 to an engine with 5W-30 (or vice versa) is the right call.
  2. Similar Oil Types: For the best results, try to mix synthetic with synthetic or conventional with conventional. While you can mix them, you’ll dilute the benefits of the superior oil.
  3. Small Quantities: Adding one quart to a system that holds five or six quarts will only have a minor impact on the overall viscosity. Your engine will not notice the difference in the short term.
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Situations to Avoid Mixing

While mixing isn’t an engine-killer, there are times when you should stick strictly to the manufacturer’s recommendation. These are key mix 5w30 and 5w40 best practices to remember.

  • Vehicles Under Warranty: Using the wrong oil grade can give a dealership grounds to deny a warranty claim for engine-related issues. It’s not worth the risk on a new car.
  • High-Performance & Turbocharged Engines: Engines with turbos or superchargers are built with extremely tight tolerances and are very sensitive to oil viscosity. Stick to the book.
  • Variable Valve Timing (VVT) Systems: Many modern engines use VVT systems that rely on specific oil pressure and flow rates to function correctly. Using oil that’s too thick can sometimes impact their performance.
  • A Full Oil Change: Never intentionally mix grades for a full oil change. You’re creating a custom blend with unknown properties. Always use a single, recommended grade.

How to Mix 5W-30 and 5W-40: A Practical Guide

If you’ve determined you’re in a situation where mixing is the best option, here’s how to mix 5w30 and 5w40 correctly and safely. This process is all about being careful and precise.

The “Top-Off” Scenario: Step-by-Step

Let’s walk through the most common scenario: topping off a low engine.

  1. Park on Level Ground: Ensure your vehicle is on a flat surface for an accurate reading. Let the engine cool for at least 15-20 minutes so the oil can settle back into the pan.
  2. Check the Dipstick: Pull the engine oil dipstick, wipe it clean with a rag, re-insert it fully, and pull it out again. Check the level against the “MIN” and “MAX” or “ADD” and “FULL” markings.
  3. Confirm You’re Low: If the oil level is at or below the “ADD” line, you need to add oil. The distance between the lines usually represents one quart (or one liter).
  4. Add Oil Slowly: Open your oil filler cap. Using a clean funnel, pour in about half a quart of the different viscosity oil (e.g., the 5W-40).
  5. Wait and Re-Check: Give the oil a minute to drain down into the pan. Re-check the dipstick. Add more oil in small increments until you reach the “FULL” line. Do not overfill!
  6. Plan Your Next Oil Change: Make a mental note (or a real one!) that you have mixed oil in your engine. It’s a good idea to perform your next scheduled oil change on time, or even a little early.

What’s the Final Viscosity?

When you mix 5W-30 and 5W-40, you create a new blend. A 50/50 mix won’t magically become “5W-35,” but the resulting oil will be thicker than a 30-grade and thinner than a 40-grade at operating temperature.

For a top-off (e.g., adding one quart to a five-quart system), the change is minimal. You might go from a 30-weight oil to what is effectively a 32-weight oil. Your engine is designed with enough tolerance to handle this small variation without any issue.

Potential Benefits and Common Problems with Mixing 5W-30 and 5W-40

While mixing is primarily for emergencies, it’s worth exploring the potential outcomes, both good and bad. This will help you understand the full picture.

Are There Any Real Benefits of Mixing 5W-30 and 5W-40?

The number one benefit is preventing catastrophic engine damage from oil starvation. That’s the big one.

However, some enthusiasts and mechanics in specific niches might create custom blends. For example:

  • Hot Climates or Track Use: An owner of an older vehicle that specifies 5W-30 might add a quart of 5W-40 in the summer for a bit more high-temperature protection during aggressive driving or towing.
  • Slightly Worn Engines: An engine that’s starting to consume a little oil might benefit from a slightly thicker blend, which can slow down consumption between changes.
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EngineNeeds Pro Tip: This is an advanced technique. For 99% of drivers, sticking to the manufacturer’s recommendation is the best path. These are not general benefits of mix 5w30 and 5w40 for everyday use.

Common Problems with Mixing 5W-30 and 5W-40

The risks are generally low but worth noting. The most common problems with mix 5w30 and 5w40 are subtle performance changes rather than outright damage.

  • Reduced Fuel Economy: If you add thicker 5W-40 to a 5W-30 system, the engine has to work slightly harder to pump it. This can result in a very minor decrease in MPG.
  • Altered Additive Package: Each oil has a unique additive package (detergents, anti-wear agents, etc.). While compatible, mixing them means you aren’t getting the precise, engineered balance of either original formula.
  • Warranty Complications: As mentioned, this is the biggest financial risk. If you have a new car, just buy the right bottle of oil.

The Eco-Friendly Angle: A Sustainable Mix 5W-30 and 5W-40 Approach

Believe it or not, there’s a sustainability aspect to this topic. A responsible car owner is an eco-friendly one, and that includes managing resources wisely.

The most sustainable mix 5w30 and 5w40 practice is using what you have. Instead of letting a half-used bottle of 5W-40 sit on your shelf for years, using it to top off your 5W-30 engine is a great way to reduce waste.

This prevents you from buying a new plastic bottle and ensures the oil gets used rather than becoming a disposal problem. Of course, the golden rule of eco-friendly car care still applies: always recycle your used motor oil. Take it to an auto parts store or a local municipal recycling center.

Frequently Asked Questions About Mixing 5W-30 and 5W-40

What happens if I accidentally did a full oil change with 5W-40 instead of 5W-30?

For one oil change interval, you are very unlikely to cause any harm. Your engine will be well-protected, though you might notice a tiny dip in fuel economy. Simply switch back to the correct 5W-30 on your next oil change.

Can I mix different brands of 5W-30 and 5W-40 oil?

Yes. As long as both oils meet the API service specification required by your vehicle’s manufacturer (e.g., API SP), they are designed to be compatible and mixable, regardless of brand.

How long can I drive with mixed oil in my engine?

You can safely drive until your next scheduled oil change. There is no need to panic and drain the oil immediately. However, if it gives you peace of mind, changing the oil a thousand miles early is a perfectly fine and cautious approach.

We hope this definitive mix 5w30 and 5w40 guide has cleared up any confusion. The key takeaway is that in a pinch, mixing is safe and smart. It’s always better to have a full sump of slightly-off-spec oil than to run low.

For your routine maintenance, however, stick with what your owner’s manual says. That’s the recipe for a long and healthy engine life.

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

Robert Lozano
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