Mix 5W40 With 0W 40- Guide to Safe
In short, yes, you can mix 5w40 with 0w40 oil in a pinch, especially for a small top-off. Both are synthetic oils with the same operating temperature viscosity, so they are generally compatible. However, it’s not recommended for a full oil change, as it creates an unpredictable blend for cold-start performance.
Picture this: you’re checking your oil before a weekend trip, and the dipstick shows you’re a quart low. You check your garage shelf, but the only bottle you have is 0W-40, and your engine calls for 5W-40. We’ve all been in a similar spot, facing a confusing decision with conflicting advice online.
Don’t worry. This guide cuts through the noise and forum debates to give you a clear, expert answer. We’ll explain exactly what happens when you decide to mix 5w40 with 0w 40, so you can make an informed choice for your engine’s health.
By the end of this article, you’ll understand what those numbers on the bottle mean, the specific scenarios where mixing is acceptable, and the best practices to follow. You’ll have the confidence to handle that low-oil situation without second-guessing yourself.
Understanding Oil Viscosity: What Do 0W-40 and 5W-40 Really Mean?
Before we can talk about mixing oils, we need to quickly decode the numbers on the bottle. This rating, set by the SAE (Society of Automotive Engineers), tells you about the oil’s viscosity—or its resistance to flow.
The “W” Number: Winter Viscosity Explained
The first number followed by a “W” (which stands for Winter, not weight) indicates the oil’s viscosity at cold temperatures. The lower this number, the better the oil flows when your engine is cold.
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0W oil flows better at very low temperatures than 5W oil.
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5W oil is slightly thicker at the same cold temperature.
This is crucial because most engine wear happens during a cold start, before the oil has fully circulated. A lower “W” number means the oil gets to vital engine components faster in freezing weather, providing better protection.
The Second Number: Operating Temperature Viscosity
The second number (in this case, “40”) represents the oil’s viscosity once the engine has warmed up to its normal operating temperature (around 212°F or 100°C).
Since both 0W-40 and 5W-40 share the same “40” rating, they behave identically once your engine is hot. They provide the same level of protection and film strength under normal running conditions.
The Big Question: Can You Safely Mix 5W40 with 0W-40?
Now for the main event. You’re standing there with two different bottles, and your engine is thirsty. Here’s the practical, no-nonsense answer you need.
The Short Answer: Yes, in a Pinch
For an emergency top-off of a quart or less, mixing 5W-40 and 0W-40 is perfectly acceptable. Because they are both modern synthetic oils with the same high-temperature viscosity, they are miscible (meaning they will mix without separating) and won’t harm your engine.
Topping off a 5W-40 system with 0W-40 is generally safer than the other way around, as you’re only improving the cold-flow properties slightly.
What Happens Inside Your Engine When You Mix?
When you add a quart of 0W-40 to an engine filled with 5W-40, you’re essentially creating a new custom blend. The resulting oil will have a cold-start viscosity somewhere between 0W and 5W.
The operating viscosity remains a “40,” so your engine is still protected when it’s hot. The main change is a minor improvement in how quickly the oil circulates on a cold morning. This is one of the very few minor benefits of mix 5w40 with 0w 40.
A Practical Guide: How to Mix 5W40 with 0W 40 Safely
Knowing you can do something is one thing; knowing how is another. This simple mix 5w40 with 0w 40 guide walks you through the right way to handle it, focusing on the most common scenario.
Scenario 1: The Emergency Top-Off
This is the most likely reason you’ll be mixing oils. Your engine is low, and you only have one of the two viscosities on hand. Follow these steps for a safe top-off.
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Check Your Oil Level: Make sure your car is on level ground and the engine has been off for at least 5-10 minutes. Pull the dipstick, wipe it clean with a rag, reinsert it fully, and pull it out again to get an accurate reading.
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Confirm How Much You Need: The marks on your dipstick usually indicate a one-quart (or one-liter) range. If the level is at the “Add” line, you need one quart.
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Add the Oil Slowly: Open your oil filler cap. Using a clean funnel, pour in about half a quart of the new oil. Wait a minute for it to settle into the oil pan.
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Re-Check the Level: Check the dipstick again. Add more oil in small increments until you reach the “Full” mark. Do not overfill your engine, as this can cause serious damage to seals and gaskets.
Scenario 2: Mixing for a Full Oil Change? (Not Recommended)
While topping off is fine, we strongly advise against intentionally mixing 0W-40 and 5W-40 for a full oil change. Each oil has a precisely formulated additive package (detergents, anti-wear agents, etc.). Mixing them in large quantities dilutes these packages and creates an oil with unknown properties.
Always use a single, manufacturer-recommended viscosity for a complete oil change. It’s the simplest way to guarantee your engine gets the protection it was designed for.
Potential Benefits and Common Problems of Mixing Oil Viscosities
While generally safe for emergencies, it’s important to understand the full picture. There are very few upsides and some potential downsides to be aware of.
The Limited “Benefits”
The only real “benefit” is convenience—using what you have on hand to avoid running your engine on low oil, which is far more dangerous. You might get a marginal improvement in cold-start performance, but it won’t be noticeable in most climates.
Common Problems with Mix 5W40 with 0W 40
Here are the potential issues, especially if you mix large quantities or do it frequently:
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Unpredictable Performance: You no longer have a true 5W-40 or 0W-40. You have a custom blend, and you can’t be certain of its exact performance characteristics in extreme cold.
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Potential Warranty Issues: If your vehicle is under warranty and experiences an engine failure, the manufacturer could deny the claim if they find you’ve been using a mixed or non-specified oil viscosity.
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Diluted Additives: While both oils have excellent additive packs, they may not be designed to work together optimally. You could slightly reduce the effectiveness of detergents or anti-wear agents.
Best Practices and Pro Tips for Oil Management
To avoid being in this situation in the first place, follow this simple mix 5w40 with 0w 40 care guide. A little preparation goes a long way.
Always Prioritize Your Owner’s Manual
Your vehicle’s manufacturer has spent millions on research to determine the perfect oil for your engine. The owner’s manual is your ultimate source of truth. Stick to the recommended viscosity and API (American Petroleum Institute) service rating.
Keep a Quart of the Correct Oil On Hand
The easiest solution is prevention. After your next oil change, buy an extra quart of the correct oil and keep it in your trunk or garage. A simple car care kit with a funnel, rags, and a quart of oil can be a lifesaver.
A Note for Off-Roaders and Performance Drivers
If you push your vehicle hard—whether on the trail, track, or towing a heavy load—engine protection is paramount. These high-stress conditions demand a consistent and robust oil film. For this reason, always stick to a single, high-quality oil viscosity. Don’t introduce variables by mixing.
The Eco-Friendly Angle: Sustainable Oil Management
Believe it or not, there are eco-friendly mix 5w40 with 0w 40 considerations. Making smart choices about oil can reduce waste and environmental impact.
Reducing Waste: Using What You Have
In an emergency, using the quart of 0W-40 you already own instead of buying a new bottle of 5W-40 is a form of waste reduction. You’re preventing an open, half-used bottle from sitting on a shelf for years. This is a practical approach to a sustainable mix 5w40 with 0w 40 scenario.
The Right Way to Recycle Used Oil
Never, ever pour used motor oil down a drain or on the ground. A single quart of oil can contaminate thousands of gallons of water. Always drain your used oil into a sealed container and take it to be recycled.
Most auto parts stores (like AutoZone, Advance Auto Parts, etc.) accept used motor oil for free. Your local municipal recycling center is another great option.
Frequently Asked Questions About Mixing 5W-40 and 0W-40 Oil
What’s the main difference between 0W-40 and 5W-40?
The only difference is their viscosity (flowability) at cold temperatures. 0W-40 flows better than 5W-40 in freezing conditions, offering slightly faster protection on a cold start. At normal operating temperatures, they perform identically.
Can I mix different brands of 5W-40 and 0W-40?
Yes, as long as both oils meet the same API service specification (e.g., API SP) listed in your owner’s manual. Modern oils are designed to be compatible. However, for best performance, it’s always ideal to stick with the same brand and viscosity.
How much 0W-40 can I add to 5W-40?
A good rule of thumb is to limit mixing to one quart for a top-off in a typical 5-quart system. This ensures the original oil’s properties are not significantly altered. Avoid mixing any more than that.
Will mixing these oils void my car’s warranty?
It potentially could. If an engine problem arises that can be linked to lubrication, a manufacturer might use the presence of a mixed-viscosity oil to deny a warranty claim. To be safe, always use the specific oil grade recommended in your manual while your vehicle is under warranty.
So, can you mix 5w40 with 0w 40? The answer is a qualified yes. It’s a safe and practical solution for an emergency top-off to get you back on the road. However, it’s not a long-term strategy or a substitute for a proper oil change with the manufacturer-recommended fluid.
The best practice is always to use the single, correct viscosity your engine was designed for. Keep an extra quart on hand, check your oil level regularly, and you’ll rarely find yourself in a bind. Keep those engines happy, stay safe on the road and trail, and never stop learning about your machine!
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