Can You Mix 5W40 And 10W40

ByRobert Lozano

In an emergency, yes, you can mix 5W-40 and 10W-40 motor oil, provided they are the same type (e.g., synthetic with synthetic). This is far better than running your engine with low oil. However, this should only be a temporary solution, as it will alter the oil’s cold-start performance, and you should plan for a full oil change soon after.

Picture this: you’re miles from home on a road trip, or deep on a favorite trail, and that dreaded low oil pressure light flickers on. You pull over, check the dipstick, and confirm your engine is dangerously low on oil. The only bottle available at the nearest stop is 10W-40, but your engine calls for 5W-40. It’s a classic DIY dilemma.

This is a situation that can make even experienced drivers second-guess themselves. We’re here to cut through the noise and give you the clear, expert answer you need to protect your engine without panic. This guide will provide a straightforward answer to the question, “can you mix 5w40 and 10w40?”

We’ll break down exactly what those numbers on the bottle mean, explore the real-world consequences of mixing these viscosities, and give you a practical, step-by-step guide for handling this exact scenario. You’ll walk away with the confidence to make the right call and understand the best practices for your engine’s health.

Decoding the Numbers: What Do 5W-40 and 10W-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 random; they’re a grade established by the SAE (Society of Automotive Engineers) that tells you the oil’s viscosity, or its resistance to flow.

The “W” is for Winter

The first number in the sequence, followed by the “W,” indicates the oil’s viscosity at cold temperatures. Think of it as the oil’s performance during a cold start.

  • 5W oil is less viscous (thinner) at low temperatures than 10W oil.
  • 10W oil is more viscous (thicker) at low temperatures than 5W oil.

A lower “W” number means the oil will flow more easily to lubricate critical engine components when you first start your car, especially in a cold climate. This is crucial for preventing wear and tear during the engine’s most vulnerable moments.

The Operating Temperature Viscosity

The second number—in this case, “40”—tells you the oil’s viscosity at the engine’s normal operating temperature (measured at 100°C or 212°F). It represents how the oil behaves once your engine is fully warmed up and you’re driving down the road.

In our scenario, both 5W-40 and 10W-40 have the exact same viscosity once the engine is hot. They will protect your engine equally well under normal running conditions.

The Critical Difference

So, the only functional difference between 5W-40 and 10W-40 is their performance during cold starts. The 5W-40 will provide slightly better flow and protection on a frosty morning, while the 10W-40 will be a bit thicker and slower to circulate initially.

So, Can You Mix 5W-40 and 10W-40? The Short and Long Answer

Now for the main event. You’re standing there with a bottle of 10W-40 and an engine that needs 5W-40. Here is the definitive answer, broken down into the emergency situation and long-term care.

The Emergency “Yes”: When It’s Okay to Mix

In an emergency, when your oil level is critically low, it is absolutely better to add the wrong viscosity oil than to run the engine on low oil. Running an engine with insufficient oil can lead to catastrophic failure, costing you thousands in repairs.

Adding a quart of 10W-40 to a system that mostly contains 5W-40 will not harm your engine. It will simply raise the overall cold viscosity slightly. Your new oil mixture will behave something like a “7.5W-40″—not a real grade, but you get the idea. This is a perfectly acceptable temporary fix to get you safely to your destination or a repair shop.

The most important rule here is to match the oil type. If your car uses full synthetic 5W-40, you must top it off with a full synthetic 10W-40. The same goes for conventional oil. Mixing synthetic and conventional isn’t ideal as you dilute the benefits of the synthetic oil.

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The Long-Term “No”: A Look at Best Practices

While mixing is fine for a top-up, you should not make a habit of it or use a mixed blend for a full oil change. This is one of the common problems with can you mix 5w40 and 10w40; people assume it’s a permanent solution. Your vehicle’s manufacturer specifies a certain viscosity for a reason.

Consistently using a thicker cold-viscosity oil than recommended can lead to:

  • Increased Cold-Start Wear: In very cold climates, the thicker oil might not reach critical parts like the valvetrain as quickly.
  • Reduced Fuel Economy: The engine has to work slightly harder to pump thicker oil, which can have a minor impact on your MPG.
  • Potential Warranty Issues: For newer vehicles still under warranty, failing to use the manufacturer-specified oil could potentially cause issues with a warranty claim down the line.

Think of mixing as a temporary patch. The goal is to restore your engine’s oil to the correct specification at your earliest convenience.

A Practical Guide: How to Mix 5W-40 and 10W-40 Safely

If you find yourself in this situation, don’t just dump the oil in and hope for the best. Following this simple can you mix 5w40 and 10w40 guide will ensure you do it right.

Step 1: Assess the Situation

First, confirm it’s a true emergency. Is the oil level below the “add” mark on your dipstick? If so, you need to add oil. Note the current weather. If it’s a hot summer day, the difference between 5W and 10W is practically meaningless. If it’s below freezing, be aware that your next cold start might be a bit rougher.

Step 2: Match the Oil Type and Certifications

This is the most critical step. Look at the back of the oil bottle. Does it say “Full Synthetic,” “Synthetic Blend,” or “Conventional”? Match this to the oil that’s already in your engine. Also, look for the API (American Petroleum Institute) or ACEA (European Automobile Manufacturers’ Association) “starburst” or “donut” symbols. Ensure the new oil meets or exceeds the specifications listed in your owner’s manual.

Step 3: Add the Oil Correctly

With the engine off and parked on a level surface, unscrew the oil filler cap (it usually has an oil can symbol on it). Using a clean funnel, pour in about half a quart of the new oil. Wait a minute for it to settle, then re-check the dipstick. Add more as needed until the level is between the “add” and “full” marks. Do not overfill the engine.

Step 4: Plan Your Next Oil Change

You’ve successfully addressed the emergency. Now, make a plan. Treat the mixed oil as a temporary measure. We recommend scheduling a full oil and filter change within the next 500 to 1,000 miles to flush out the mixed-viscosity blend and restore your engine’s lubricant to factory specs. This is one of the most important can you mix 5w40 and 10w40 tips to remember.

The Impact on Your Vehicle: From Daily Drivers to Off-Road Rigs

How much does this really matter for your specific vehicle? The impact can vary depending on how you use it.

For the Daily Commuter

For a standard car, truck, or SUV used for daily driving, a one-time mix to top off is very low-risk. The biggest noticeable difference might be a slightly longer crank time on a freezing winter morning. For most drivers, the difference will be completely unnoticeable.

For the Performance Enthusiast & DIY Mechanic

If you have a high-performance, turbocharged, or supercharged engine, oil specifications are much more critical. These engines operate under higher stress and temperatures. While a top-off is still better than running low, you should be extra diligent about getting a proper oil change as soon as possible to maintain peak performance and protection.

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For the Off-Roader and Tow Rig

When you’re rock crawling or towing a heavy load, your engine is working hard. The good news is that both 5W-40 and 10W-40 protect equally well once the engine is hot. The main concern for off-roaders is the initial cold start at the trailhead. The bigger danger, by far, is running low on oil on an incline, which can cause oil starvation. Topping off, even with a mixed viscosity, is the right call every time.

Sustainable and Eco-Friendly Oil Practices

Part of being a responsible vehicle owner is handling fluids properly. When considering a sustainable can you mix 5w40 and 10w40 approach, the goal is to reduce waste and environmental impact.

Intentionally mixing different oils just to use up old bottles is not a recommended practice. The most eco-friendly can you mix 5w40 and 10w40 strategy is to use the correct oil for its full service interval, which maximizes your engine’s efficiency and minimizes waste.

When you do perform your oil change, never dump the old oil. Whether it’s mixed or not, used motor oil is a hazardous material. Collect it in a drain pan and transfer it to a sealed container. Nearly all auto parts stores and many municipal recycling centers accept used motor oil for free.

Frequently Asked Questions About Mixing 5W-40 and 10W-40 Oil

What happens if I accidentally mixed a full quart of 10W-40 into my 5W-40?

Don’t panic. Assuming your engine holds around 5 quarts of oil, one quart of a different cold viscosity will not cause any damage. Your oil’s cold-start properties will be slightly thicker, but its protection at operating temperature remains the same. Just plan to get back to the recommended grade at your next oil change.

Can I mix different brands of 5W-40 and 10W-40?

Yes. As long as both oils meet the API/ACEA specifications required by your vehicle and are the same type (synthetic with synthetic), it is safe to mix brands. Modern motor oils are formulated to be compatible with each other.

Does mixing 5W-40 and 10W-40 void my car’s warranty?

Using the wrong oil grade *could* be grounds for a manufacturer to deny a warranty claim for a related engine failure. However, it’s highly unlikely that a one-time emergency top-off would be detectable or cause an issue. To be safe, always revert to the specified grade as soon as possible.

Is it better to mix viscosities or run with low oil?

This is the easiest question to answer: It is always, 100% of the time, better to top off with a compatible but different viscosity oil than to continue driving with the oil level below the minimum mark. Low oil level is a direct path to severe engine damage.

Your Engine’s Health Comes First

So, the final verdict is clear. While not ideal for the long haul, mixing 5W-40 and 10W-40 is a perfectly safe and smart decision in a pinch. The potential negative effects are minimal and temporary, whereas the consequences of running on low oil are severe and permanent.

The core of smart vehicle maintenance is understanding the rules and knowing when it’s okay to bend them for the greater good of your engine. By remembering to match the oil type (synthetic/conventional) and planning for a proper oil change soon after, you can handle this common roadside problem like a pro.

Drive smart, stay prepared, and keep that engine humming for years to come.

Robert Lozano
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