Can You Mix 5W40 With 15W40 – 40 – A Technician’S Guide To Emergency

ByRobert Lozano

You can mix 5W-40 with 15W-40 oil in an emergency to top off a low engine, as mixing is always better than running on dangerously low oil. However, this should only be a temporary solution, as it will alter the oil’s viscosity and reduce its cold-start performance, potentially leading to increased engine wear over time.

You’re halfway through a road trip, or maybe deep on a favorite trail, when that dreaded low oil pressure light flickers on. You pull over, pop the hood, and the dipstick confirms it: you’re low. The only oil at the nearest gas station is 15W-40, but your engine calls for 5W-40. The panic sets in.

We’ve all been there. This is a common dilemma for drivers, riders, and off-roaders alike. The question of can you mix 5W-40 with 15W-40 isn’t just academic; it’s a real-world problem that needs a clear, no-nonsense answer.

In this complete guide, we’ll break down exactly what happens when you mix these two oil weights. We’ll cover the science, the risks, the right way to handle an emergency top-off, and how to get your engine back to optimal health afterward. Let’s get you back on the road with confidence.

Decoding the Numbers: What 5W-40 and 15W-40 Really Mean

Before we can talk about mixing, we need to understand what we’re working with. Those numbers on the oil bottle aren’t random; they represent the oil’s viscosity, or its resistance to flow, at different temperatures. This is governed by the Society of Automotive Engineers (SAE) J300 standard.

The “W” is for Winter

The first number followed by a “W” (e.g., 5W or 15W) indicates the oil’s viscosity in cold conditions. Think of it as the “Winter” rating. The lower this number, the better the oil flows when the engine is cold.

  • 5W-40: This oil is thinner and flows more easily at low temperatures. It allows the engine to turn over quickly and lubricates critical components almost instantly on a cold start.
  • 15W-40: This oil is thicker at the same low temperatures. It flows more slowly, which can put a momentary strain on the engine during a cold start, especially in freezing weather.

Imagine pouring two syrups from the fridge. The 5W-40 is like warm maple syrup, flowing easily. The 15W-40 is more like cold honey—thicker and slower to move.

The Operating Temperature Viscosity

The second number (40 in this case) represents the oil’s viscosity once the engine has reached its normal operating temperature (around 212°F or 100°C).

In our scenario, both oils are designed to have the same viscosity grade when hot. This is a key point. Once your engine is warmed up, both a 5W-40 and a 15W-40 oil provide a similar level of protection.

So, Can You Mix 5W-40 with 15W-40? The Short and Long Answer

Now for the main event. You’re standing there with a quart of 15W-40 in your hand, looking at an engine that needs 5W-40. Here is the practical advice you need, based on decades of experience in the shop and on the trail.

The Short Answer: Yes, in an Emergency

Let’s be crystal clear: running your engine with dangerously low oil is far worse than topping it off with a slightly different viscosity oil. Modern engine oils, as long as they meet the same API (American Petroleum Institute) service standard (like API SN, SP, or CK-4), are formulated to be compatible with each other.

Adding a quart of 15W-40 to an engine filled with 5W-40 will not cause it to sludge up, turn to goo, or immediately destroy your engine. It will simply get you safely to your destination or the nearest auto parts store.

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The Long Answer: It’s a Compromise, Not a Solution

While it’s safe for a short-term fix, mixing these oils is not a recommended long-term practice. When you mix them, you create a new blend with an unknown cold-start viscosity. The final “W” rating will land somewhere between 5W and 15W.

This means you are compromising the most important job of the 5W-40: its ability to provide excellent lubrication during cold starts. This is one of the most common problems with can you mix 5w40 with 15w40—you lose the specific benefit you paid for.

Potential Risks and Common Problems From Mixing Oil Weights

Understanding the consequences will help you appreciate why this is a temporary fix. This isn’t about scaremongering; it’s about smart engine care. Here are the real-world impacts.

Reduced Cold-Start Protection

The vast majority of engine wear happens in the first few seconds after you turn the key. During this time, the oil needs to rush from the oil pan to the top of the engine to lubricate parts like the valvetrain and camshafts.

Your new 5W/15W blend will be thicker on startup than pure 5W-40. This means it will take fractions of a second longer to reach those critical parts, leading to a tiny bit of extra wear each time you start the engine from cold. Over time, this adds up.

Impact on Variable Valve Timing (VVT) Systems

Many modern engines use VVT systems that rely on oil pressure to function correctly. These systems are highly sensitive to oil viscosity, especially when cold. Using a thicker oil blend can sometimes cause sluggish VVT performance, potentially triggering a check engine light or causing rough running until the engine warms up.

Potential Warranty Issues

Your vehicle’s manufacturer specifies an oil viscosity for a reason. They’ve done extensive testing to find the optimal grade for performance, protection, and fuel economy. Using an oil (or a mix) that falls outside their recommendations could, in a worst-case scenario, give them grounds to deny a warranty claim related to the engine.

A Practical Guide: How to Mix 5W-40 with 15W-40 in an Emergency

If you find yourself in a bind, don’t panic. Follow this simple can you mix 5w40 with 15w40 guide to handle the situation like a pro and minimize any potential issues.

The Best Practices for an Emergency Top-Off

  1. Confirm the Need: Park your vehicle on a level surface. Wait at least 10-15 minutes for the oil to drain back into the pan for an accurate reading.
  2. Read the Dipstick: Pull the dipstick, wipe it clean with a rag, fully re-insert it, and pull it out again. If the level is at or below the “ADD” or “MIN” line, you need to add oil.
  3. Add Sparingly: Your goal is not to fill it to the “FULL” mark. Your goal is to get it safely within the operating range. Start by adding just half a quart (about half the bottle).
  4. Wait and Re-Check: Give the oil a minute to settle, then check the dipstick again. Add a little more if needed to bring it to the halfway point between “ADD” and “FULL.” Do not overfill!
  5. Match the Type, If Possible: If your car uses full synthetic 5W-40, try to use a full synthetic 15W-40. If it uses conventional, use conventional. This helps keep the additive packages as compatible as possible.
  6. Plan Your Next Move: The most crucial step. Treat this as a temporary patch. Plan to get a full oil and filter change with the correct 5W-40 oil as soon as you reasonably can.

Sustainable and Eco-Friendly Oil Management

Being a responsible DIYer also means thinking about the environment. This topic provides a great opportunity to discuss a sustainable can you mix 5w40 with 15w40 approach.

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Mixing oils in an emergency is one thing, but creating waste is another. Avoid buying the wrong oil in the first place by always checking your owner’s manual. Having a spare, correct-grade quart of oil in your trunk is the best preventative measure.

When you perform your follow-up oil change, always recycle your used motor oil and the old oil filter. Most auto parts stores, and many county recycling centers, will accept used oil for free. This is a simple, eco-friendly can you mix 5w40 with 15w40 practice that keeps harmful contaminants out of our groundwater.

Frequently Asked Questions About Mixing 5W-40 and 15W-40

What happens if I accidentally put a full quart of 15W-40 in my 5W-40 engine?

Don’t worry, your engine will be fine in the short term. A single quart in a system that holds 5-6 quarts will thicken the overall blend, but not drastically. Your cold-start protection will be slightly reduced, so just be sure to schedule a proper oil change within the next few hundred miles to restore the correct viscosity.

Can I mix 5W-40 and 15W-40 in my diesel truck?

Yes, the same logic applies, and it’s even more common in the diesel world. Many heavy-duty diesel engines use 15W-40 as a standard, while some newer or modified ones use 5W-40 for better cold-weather performance. As long as both oils meet the correct API “C” rating (like CK-4), you can mix them in a pinch. However, the cold-start benefits of the 5W-40 will be diminished.

How soon should I get an oil change after mixing weights?

There’s no magic number, but a good rule of thumb is to get it done at your earliest convenience, ideally within 500 to 1,000 miles. Don’t push it to the next full oil change interval. Think of it as restoring your engine’s proper diet after a “cheat meal.”

Is it better to run with very low oil or top off with the wrong weight?

Always top off. Running an engine on critically low oil can lead to catastrophic failure due to oil starvation. The risks of mixing oil weights are minimal and long-term, while the risk of running low on oil is immediate and severe.

Your Engine, Your Call: The Final Word

So, can you mix 5W-40 with 15W-40? The answer is a confident “yes” for emergency situations. It’s a practical solution that prioritizes engine survival over perfect specifications.

The key takeaway from this can you mix 5w40 with 15w40 care guide is to understand that it’s a temporary fix. You are making a conscious trade-off, sacrificing some cold-start performance for the immediate and critical need for lubrication. By adding just enough to get by and scheduling a proper oil change soon after, you can navigate a tricky situation without causing any harm to your engine.

Keep the correct oil on hand when you can, but never hesitate to use what’s available when the alternative is running dry. Keep those engines happy and hit the road with confidence!

Robert Lozano
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