Can You Mix 20W50 With 15W40 – A Practical Guide For Emergencies
In an emergency, you can mix 20W-50 and 15W-40 oil to top off your engine. It is far better than running on dangerously low oil. However, this should only be a temporary solution, as the resulting oil will have an unpredictable viscosity that doesn’t perfectly match your engine’s requirements.
You’re on a long stretch of highway or deep on a trail, and the dreaded oil light flickers. You pull over, pop the hood, and the dipstick confirms your fear: you’re dangerously low on oil. A search of your trunk reveals a dusty quart of 20W-50, but your engine calls for 15W-40. The panic sets in.
This is a situation every driver, rider, and DIY mechanic dreads. Making the wrong choice could mean serious trouble for your engine. We promise to give you the clear, no-nonsense answer to the question, can you mix 20W-50 with 15W-40, backed by real-world mechanical know-how.
In this complete guide, we’ll break down what those numbers on the bottle actually mean, explore the specific scenarios where mixing is acceptable, outline the potential risks, and give you a step-by-step process for doing it safely when you have no other choice.
Understanding the Numbers: What Do 20W-50 and 15W-40 Really Mean?
Before we can talk about mixing oils, we need to understand what we’re working with. Those numbers on the bottle aren’t just for decoration; they represent the oil’s viscosity, which is its resistance to flow.
Think of it like this: honey has a high viscosity (it’s thick), and water has a low viscosity (it’s thin). Your engine needs an oil that’s thin enough to flow quickly on a cold start but thick enough to protect components when it’s hot.
The “W” Number: Cold Start Performance
The first number followed by a “W” (which stands for “Winter”) tells you how the oil flows when it’s cold. The lower the “W” number, the better it flows in cold temperatures.
- 15W: This oil is thinner and flows more easily at cold startup temperatures than a 20W oil.
- 20W: This oil is slightly thicker when cold.
This is critical because most engine wear happens during the first few seconds after a cold start, before the oil has had a chance to circulate fully. A lower “W” number means faster protection for crucial parts like your camshafts and bearings.
The Second Number: High-Temperature Protection
The second number indicates the oil’s viscosity at normal operating temperatures (measured at 100°C or 212°F). A higher number means the oil remains thicker and more protective under high heat and heavy loads.
- 40: This is a common viscosity for many modern gasoline and diesel engines, offering a great balance of protection and efficiency.
- 50: This is a thicker oil, often used in high-performance, air-cooled, or older engines that may have larger bearing clearances or run hotter.
Conventional vs. Synthetic: The Base Oil Matters
It’s also important to know if your oil is conventional (refined from crude oil) or synthetic (man-made for higher performance). The good news is that all modern oils that meet the American Petroleum Institute (API) standards are designed to be compatible. You can mix conventional with synthetic, but you’ll dilute the benefits of the more expensive synthetic oil.
So, Can You Mix 20W-50 with 15W-40? The Short and Long Answer
Here’s the straight answer you came for, broken down into the simple version and the more detailed explanation every good DIYer should know. This section serves as a core part of our can you mix 20w50 with 15w40 guide.
The Short Answer: Yes, in a Pinch
Absolutely. If your choice is between mixing different weights or running your engine on dangerously low oil, mix the oil. The potential damage from oil starvation caused by low pressure is catastrophic and far outweighs the risks of a temporary, slightly-off-spec oil viscosity.
The Long Answer: It’s a Compromise, Not a Solution
When you mix 20W-50 with 15W-40, you don’t get to choose the best properties of each. Instead, you create a new oil blend with an intermediate viscosity. If you mix them in equal parts, you’ve essentially created a hypothetical “17.5W-45” oil.
This new grade isn’t what your engine’s engineers designed the internal clearances and oil passages for. While it will work, it’s not optimal. The cold-start flow will be slower than 15W-40, and the high-temperature protection will be less robust than 20W-50.
The “Better Than Nothing” Rule: Low Oil is Your Real Enemy
Let’s be crystal clear: low oil level is the real danger. When the oil level drops below the pickup tube in the oil pan, the oil pump will suck in air. This causes a rapid and complete loss of oil pressure.
Without oil pressure, the metal-on-metal contact inside your engine will cause catastrophic failure in a matter of seconds. Spun bearings, scored cylinder walls, and seized components are the result. A slightly incorrect oil viscosity is a minor issue compared to that.
When Is It Okay to Mix? Real-World Scenarios
While we don’t recommend mixing as a regular practice, there are specific situations where it’s a logical and safe temporary fix. Here are a few common scenarios faced by car owners and off-roaders.
The Emergency Top-Off
This is the most common and acceptable reason to mix oil weights. You’re on a road trip, in the middle of nowhere, or on an off-road trail, and you need to add a quart to get home or to the nearest auto parts store safely. In this case, adding a quart of 20W-50 to an engine filled with 15W-40 is the right call.
Older, High-Mileage Engines
Some owners of older vehicles with worn piston rings or seals find their engines consume a bit of oil. They might intentionally mix a thicker oil (like 20W-50) with their recommended grade to slightly increase the operating viscosity. This can sometimes help reduce oil consumption or quiet down noisy valve lifters. This is an advanced technique and should be done with caution.
Adjusting for Extreme Use
An off-roader about to tackle a slow, high-load rock crawling section in the summer heat might add a bit of 20W-50 to their 15W-40 to provide an extra layer of film strength. Again, this is not for daily driving and is a specific use case for enthusiasts who understand the trade-offs.
Common Problems and Risks: What to Watch Out For
Understanding the common problems with can you mix 20w50 with 15w40 is key to making an informed decision. While mixing won’t make your engine explode, it does come with some performance compromises you should be aware of.
Altered Cold Start Performance
Your new oil blend will be thicker on a cold morning than the 15W-40 your engine expects. This means it will take a fraction of a second longer for oil to reach the top of the engine. In extremely cold climates, this could contribute to minor long-term wear.
Reduced High-Temperature Protection
If your engine is designed for a thick 50-weight oil (like many air-cooled Porsches or certain motorcycles), thinning it out with a 40-weight oil reduces its ability to protect against shear and wear under extreme heat. This is especially important if you’re towing, racing, or operating in a hot climate.
Diluted Additive Packages
Every motor oil has a unique recipe of additives: detergents to keep things clean, anti-wear agents, and viscosity improvers. While the base oils are compatible, you are diluting the carefully balanced formula of both oils. This slightly reduces the overall effectiveness of the additive package.
Potential Warranty Issues
For any vehicle still under a manufacturer’s warranty, this is a big one. If you experience an engine failure and the dealership finds you’ve been using a non-specified oil grade (which a mix technically is), they could deny your warranty claim. Always stick to the OEM-recommended grade for vehicles under warranty.
A Practical Guide: How to Mix 20W-50 with 15W-40 Safely
If you find yourself in a situation where you need to mix, follow these can you mix 20w50 with 15w40 best practices. This simple care guide ensures you do it safely and minimize any potential risks.
- Confirm It’s an Emergency: First, make sure you actually need oil. Check the dipstick on a level surface after the engine has been off for a few minutes. If the level is at or below the “ADD” mark, you need to act.
- Check the API Service “Donut”: Look for the circular API seal on the back of both oil bottles. As long as they meet the same or a compatible service classification (e.g., both are “API SP” for gas engines or “API CK-4” for diesels), they are designed to be chemically compatible.
- Add the Minimum Amount Necessary: Don’t just dump the whole bottle in. Add about half a quart, wait a minute, and re-check the dipstick. Add just enough to bring the level into the safe cross-hatched area, preferably in the middle. This minimizes the impact on the overall viscosity.
- Create a Plan for an Oil Change: The most crucial step. Treat this mixed oil as a temporary fix. Plan to perform a full oil and filter change at your earliest convenience to get the correct, manufacturer-recommended oil back into your engine.
Beyond the Mix: Eco-Friendly and Sustainable Oil Practices
Part of being a responsible vehicle owner is managing fluids properly. While the question is about mixing, the best sustainable approach is to use the right oil from the start. This leads to better performance and less waste.
When you do perform that follow-up oil change, remember that motor oil is a hazardous material. Never dump used oil down a drain or on the ground. It’s one of the most significant sources of water pollution.
The best eco-friendly can you mix 20w50 with 15w40 tip is to manage the aftermath correctly. Pour your used oil into a dedicated drain pan and transfer it to a sealed container (like the empty bottle the new oil came in). Nearly all auto parts stores and many local service stations will accept used motor oil for recycling free of charge.
Frequently Asked Questions About Mixing 20W-50 and 15W-40 Oil
Can I mix different brands of 20W-50 and 15W-40?
Yes. As long as both oils meet the API service classification recommended for your vehicle, mixing brands is perfectly fine. Brand compatibility is less important than ensuring the API rating and viscosity are appropriate.
What about mixing conventional 15W-40 with synthetic 20W-50?
You can. All API-certified conventional, synthetic blend, and full synthetic oils are compatible and will mix without issue. However, you will lose the high-performance benefits (like longer drain intervals and better thermal stability) of the synthetic oil by diluting it with conventional.
How long can I drive with mixed oil in my engine?
Think of it as a temporary spare tire. It will get you home, but you shouldn’t rely on it for thousands of miles. We recommend changing the oil and filter within the next 500-1,000 miles, or at your next scheduled weekend maintenance day—whichever comes first.
Will mixing these oils cause engine sludge?
No. This is a common myth. Modern, detergent motor oils that are API-certified will not react with each other to form sludge. Sludge is the result of extreme oil degradation from overheating, moisture contamination, and massively extended oil change intervals, not from mixing compatible oils.
Your Engine, Your Call: The Final Word
So, can you mix 20W-50 with 15W-40? The answer is a resounding yes—in an emergency. It is always the better choice over letting your engine starve for oil. The key is to view it as a short-term patch, not a long-term strategy.
By understanding what viscosity means and following the best practices for topping off, you can confidently handle that stressful low-oil situation without worry. Add what you need to get back on the road safely, and then plan to give your engine the fresh, correct grade of oil it deserves as soon as you can.
Keep those wrenches turning, check your fluids regularly, and drive safe!
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