Mixing 5W30 And 5W20 – The Definitive Guide On What Happens And When

ByRobert Lozano

In a pinch, mixing 5W-30 and 5W-20 oil to top off your engine is generally safe and far better than running on low oil. However, this creates an oil with an altered viscosity that doesn’t perfectly match your engine’s specifications, so it should only be a temporary solution before a proper oil change.

You’re in the garage, dipstick in hand, and you see the dreaded news: you’re a quart low. You scan your shelf and find a fresh bottle of 5W-30, but your car’s oil cap clearly says 5W-20. The auto parts store is closed, and you have to be somewhere tomorrow.

We’ve all faced this exact dilemma. It’s a moment that sparks a frantic search, leading to confusing forum threads filled with conflicting advice. Some say it’s fine, others warn of catastrophic engine failure.

This comprehensive guide will cut through that noise. We’ll give you the straight, expert answer on the real-world consequences of mixing 5w30 and 5w20, break down exactly when it’s a last resort, and outline the best practices for keeping your engine healthy for the long haul.

Understanding the Numbers: What Do 5W-20 and 5W-30 Actually Mean?

Before we dive into mixing, let’s quickly demystify those numbers on the bottle. Understanding them is the key to making an informed decision. This isn’t just jargon; it’s your engine’s lifeblood specification.

The “W” Stands for Winter

The first part of the oil grade, the “5W,” tells you how the oil behaves in cold temperatures. The “W” stands for Winter.

A lower number here means the oil is thinner and flows more easily when it’s cold. This is crucial for starting your engine on a frosty morning, as it allows the oil to circulate quickly and lubricate vital components.

In our case, both 5W-20 and 5W-30 have the same cold-start viscosity of “5W.” This is great news because it means when it comes to cold starts, they behave identically. This shared characteristic is a big reason why mixing them in an emergency isn’t immediately catastrophic.

The Operating Temperature Viscosity: The Real Difference

The second number—the “20” or “30”—is the most important part of this discussion. This indicates the oil’s viscosity, or thickness, when your engine is at its normal operating temperature (around 212°F or 100°C).

Here’s the simple breakdown:

  • 5W-20: Is thinner at operating temperature.
  • 5W-30: Is thicker at operating temperature.

Modern engines are built with incredibly tight tolerances. Engineers specify a certain viscosity to ensure a perfect, protective film of oil exists between moving metal parts like bearings, pistons, and camshafts.

The Big Question: What Actually Happens When Mixing 5w30 and 5w20?

So, you’ve poured that quart of 5W-30 into your 5W-20 engine. What’s the immediate result? You haven’t created a toxic sludge, but you have fundamentally changed the oil’s properties. Here’s a look at the science and some common problems with mixing 5w30 and 5w20.

The Immediate Result: You Create a “Franken-Viscosity”

When you mix the two oils, you don’t get layers of thick and thin. They blend completely. The result is a new oil with a viscosity somewhere between the two.

If your engine held 5 quarts of 5W-20 and you added 1 quart of 5W-30, you’d end up with an oil that’s slightly thicker than a 20-weight but still thinner than a 30-weight. Think of it as creating a “5W-22” or “5W-23.”

While this is better than being dangerously low on oil, this new viscosity is not what your engine was designed for. It’s an unknown variable you’ve introduced into a highly engineered system.

Potential Performance and Protection Impacts

Running an oil that’s slightly thicker or thinner than specified can have subtle effects. If you add thicker 5W-30 to a 5W-20 engine, you might notice:

  • Slightly Reduced Fuel Economy: Thicker oil creates more internal friction, or “drag,” which requires the engine to work a tiny bit harder. The effect is often minimal, but it’s there.
  • Variable Valve Timing (VVT) Issues: Many modern engines use VVT systems that rely on oil pressure to function. Using an oil that’s too thick or thin can sometimes affect the precise timing of these systems, potentially leading to a check engine light in sensitive vehicles.
See also  How Much Gear Oil In Outboard – The Definitive Fill-To-Spill Guide

Conversely, adding thinner 5W-20 to a 5W-30 engine could slightly reduce the thickness of the protective oil film at high temperatures, which isn’t ideal for engines designed for that extra protection.

The Additive Package Concern

Modern motor oils are more than just base oil. They contain a complex additive package with detergents, anti-wear agents, friction modifiers, and corrosion inhibitors. The good news is that all API-certified oils are required to be compatible.

However, mixing oils from different brands can dilute the specific, fine-tuned formula of each. While not a major risk for a simple top-off, it’s another reason why sticking to one type and grade is one of the most important mixing 5w30 and 5w20 best practices.

A Practical Mixing 5w30 and 5w20 Guide for Emergencies

Theory is great, but what about the real world? Here’s a practical guide for when you’re standing over your engine with two different bottles of oil. This is how to approach mixing 5w30 and 5w20 safely.

The Emergency Top-Off: When It’s Okay

The single most important rule of engine health is that the wrong oil is better than no oil. Running an engine critically low on oil will cause catastrophic damage, period.

If you check your dipstick and you’re a quart or more low, and the only thing you have is a different viscosity (like 5W-30 instead of 5W-20), absolutely use it to top off the engine. This will get you safely back on the road, to the auto parts store, or home to your garage.

This is the one scenario where mixing is not only acceptable but recommended. The potential risk from a slightly altered viscosity is microscopic compared to the guaranteed damage from oil starvation.

What NOT to Do: The Full Oil Change Mix

Never intentionally mix 5W-20 and 5W-30 for a full oil change. If your car requires 5 quarts of 5W-20, use 5 quarts of 5W-20. Don’t try to save a few bucks by using up leftover bottles of different grades.

Starting with a fresh, correct fill of oil is the foundation of a healthy engine. It ensures your warranty remains intact and your engine gets the exact protection it was designed to have from day one.

Common Problems and Long-Term Risks of Habitual Mixing

While a one-time emergency top-off is harmless, making a habit of mixing viscosities or using the wrong oil can lead to problems down the road. These are the risks you want to avoid.

Potentially Voiding Your Vehicle’s Warranty

Your vehicle’s manufacturer specifies a particular oil viscosity for a reason. If you experience an engine failure while under warranty and the dealership discovers you’ve been using the wrong grade of oil, they have grounds to deny your claim. It’s a simple risk not worth taking.

Increased Engine Wear Over Time

An oil film that is too thin may not adequately protect against metal-to-metal contact under high load or heat. An oil film that is too thick may not flow quickly enough into tight passages, especially in modern engines with complex systems like VVT or turbochargers.

Over tens of thousands of miles, this subtle mismatch in protection can accelerate wear on critical components, potentially leading to costly repairs later in the vehicle’s life.

Best Practices: Your Care Guide After Mixing Oils

Okay, so you did it. You were in a bind and topped off with a different grade. No need to panic. Here are the simple, actionable steps to take next. Consider this your essential mixing 5w30 and 5w20 care guide.

See also  Gear Oil Is Good For Chain Lube – A Diyer’S Guide To Pros, Cons

The Golden Rule: Change It Sooner Rather Than Later

Treat the mixed oil as a temporary patch. Don’t wait for your next scheduled 5,000-mile interval. Plan to perform a full oil and filter change at your earliest convenience—within the next few hundred miles is a great rule of thumb.

This will flush out the “Franken-oil” and restore your engine’s lubrication system with the exact viscosity it needs to perform its best and stay protected.

Performing a Proper Oil Change

When it’s time to correct the mix, make sure you do it right. You’ll need:

  1. The correct amount and type of manufacturer-recommended oil.
  2. A new, high-quality oil filter (like a WIX, Mobil 1, or OEM filter).
  3. A drain pan to catch the old oil.
  4. A wrench for the drain plug and an oil filter wrench.

Warm up the engine, safely drain all the old oil, replace the filter, tighten the drain plug to the correct torque, and refill with the fresh, correct-grade oil. Always double-check the level on the dipstick after you’re done.

Sustainable and Eco-Friendly Oil Disposal

Don’t just dump that used oil! Motor oil is highly toxic and can contaminate soil and groundwater. An eco-friendly mixing 5w30 and 5w20 process ends with responsible disposal.

Pour the old oil from your drain pan into the empty oil bottles. Nearly all auto parts stores (like AutoZone, O’Reilly, or Advance Auto Parts) and many local mechanic shops will accept used motor oil for recycling, free of charge. This is a simple, sustainable step every DIYer should take.

Frequently Asked Questions About Mixing 5W-30 and 5W-20

Can I mix 5W-20 and 5W-30 synthetic oil?

Yes, the same rules apply. You can mix full synthetic 5W-20 and 5W-30 in an emergency top-off situation. All API-certified synthetic oils are compatible. However, just like with conventional oil, you should change it out for the correct grade at your earliest convenience.

What’s worse: putting 5W-30 in a 5W-20 engine, or vice versa?

Generally, putting a slightly thicker oil (5W-30) into an engine designed for a thinner oil (5W-20) is considered less risky than the other way around. The thicker oil ensures a robust protective film, even if it slightly impacts fuel economy. Putting a thinner oil in an engine designed for thicker oil could compromise lubrication under extreme heat or load.

How long can I drive with mixed oil?

There’s no exact mileage, but it should be viewed as a temporary fix. Aim to get a proper oil change within a few hundred miles, or at the very latest, by your next weekend. Don’t complete the full 5,000 or 10,000-mile oil change interval on a mixed fill.

Your Engine, Your Call: The Final Word

At the end of the day, mixing 5w30 and 5w20 isn’t the engine-destroying catastrophe some make it out to be—if it’s done as a short-term, emergency measure to avoid running on low oil.

The key takeaway is simple: always prioritize having enough oil over having the perfect oil. But once the emergency is over, prioritize getting the right oil back in your engine.

By following your manufacturer’s recommendations, you ensure your engine’s longevity, maintain its warranty, and get the performance and efficiency you expect. Check your oil regularly, use the right stuff, and keep wrenching safely!

Robert Lozano
Latest posts by Robert Lozano (see all)

Similar Posts