Can You Mix Engine Oil Brands? The Truth About Mixing Oils and What It Means for Your Engine

The short answer is: Technically, you can mix engine oil brands in a pinch, but it’s almost never advisable—and doing so long-term could cause serious damage to your engine. Let’s unpack why this is the case, break down the risks, and explore what to do if you find yourself in a situation where mixing seems unavoidable.

Why Mixing Engine Oil Brands Is Risky (Even If They’re “Compatible”)

Engine oil is far more than just a lubricant. It’s a complex blend of base oils (mineral, synthetic, or semi-synthetic) and additives—chemicals that perform critical functions like reducing friction, preventing corrosion, neutralizing acids, and keeping contaminants suspended so they can be filtered out. Different brands formulate their oils with unique additive packages tailored to specific engine types, driving conditions, and performance goals. When you mix two oils from different manufacturers, you’re essentially blending two distinct chemical cocktails. Here’s what can go wrong:

1. Additive Conflicts

Additives are the “secret sauce” of engine oil, but they don’t always play well together. For example:

• Detergents and Dispersants: These keep engines clean by preventing sludge and varnish buildup. If one oil uses a detergent that reacts poorly with another brand’s dispersant, they might cancel each other out—or worse, form a gel-like substance that clogs oil passages.
• Anti-Wear Agents: Zinc dialkyldithiophosphate (ZDDP) is a common anti-wear additive, but its concentration varies by brand and oil type (e.g., high-ZDDP oils for older engines vs. low-ZDDP for newer, catalytic converter-friendly formulations). Mixing oils with mismatched ZDDP levels could either over-protect (causing deposits) or under-protect (leading to metal-on-metal wear).
• Friction Modifiers: These reduce friction to improve fuel efficiency or quiet engine operation. If two oils use different types (e.g., molybdenum-based vs. organic), they might not work synergistically, potentially increasing wear instead of reducing it.

In some cases, these conflicts are immediate—for instance, causing the oil to lose its ability to form a protective film on engine parts. In others, the damage accumulates slowly over time, leading to costly repairs down the line.

2. Viscosity Breakdown

Viscosity (how “thick” or “thin” oil is at different temperatures) is one of the most critical specs for engine oil. It’s denoted by a rating like 5W-30, where “5W” is the cold-temperature viscosity and “30” is the hot-temperature viscosity. While most oils meet industry standards (e.g., API SN, ACEA A3/B4), different brands may achieve these ratings using different base oils or viscosity index improvers (VIIs)—chemicals that help oil maintain its viscosity across temperatures.

Mixing oils can alter the overall viscosity. For example, blending a 5W-30 with a higher-viscosity 10W-40 might result in an oil that’s too thick at startup (failing to flow quickly to critical parts) or too thin at high temperatures (unable to protect against metal friction). Even small viscosity changes can reduce the oil’s ability to seal the engine, cool components, or prevent leaks.

3. Contaminant Risks

Oils from different brands may have varying levels of contaminants or “wear metals” (tiny particles of engine material shed during operation). If one oil is contaminated with dirt, metal shavings, or water, mixing it with another oil spreads those contaminants throughout the system. Over time, this can accelerate wear on bearings, pistons, and camshafts.

Worse, some oils contain “hidden” contaminants from manufacturing—like residual solvents or processing chemicals—that aren’t listed on the label. These can react with additives in another brand’s oil, leading to unexpected breakdowns.

When Might Mixing Be Necessary? (And How to Minimize Risk)

Let’s be clear: There’s no scenario where mixing oils is “good” for your engine. However, there are rare emergency situations where it might be the lesser of two evils. For example:

• Running Low on Oil: If your oil level is critically low (e.g., due to a leak or forgetting to check), adding a small amount of a different brand to top off might prevent catastrophic engine damage—at least until you can drain and replace the oil.
• Remote Locations: If you’re traveling and can’t find your usual oil brand, you might have to use whatever is available to keep the engine running.

If you find yourself in either of these situations, here’s how to reduce risk:

1. Check for Compatibility First

Not all oils are created equal, but some are more compatible than others. Look for:

• Shared Specifications: Ensure both oils meet the same industry standards (e.g., API SP, ACEA C3) and your vehicle manufacturer’s requirements (check your owner’s manual). If one oil is API SN and the other is API SN Plus, they’re likely more compatible than an API SN and an older API SM.
• Similar Base Oils: Oils labeled “full synthetic” are generally more compatible with other full synthetics than with mineral or semi-synthetic oils. Synthetic base oils (like PAOs or esters) have more uniform chemical structures, making them less prone to additive conflicts.
• Same Viscosity Grade: Always match the viscosity (e.g., 5W-30 to 5W-30). Mixing viscosities is far riskier than mixing brands with the same viscosity.

2. Use the Smallest Amount Possible

If you’re topping off, add only enough to reach the recommended oil level—never drain half the engine and replace it with a different brand. A small amount (less than 10% of the total oil volume) is less likely to disrupt the existing additive balance.

3. Monitor Closely Afterward

After mixing, watch for warning signs:

• Unusual Noises: Knocking, grinding, or rattling from the engine could indicate insufficient lubrication.
• Reduced Performance: Sluggish acceleration, poor fuel economy, or rough idling might signal oil breakdown.
• Oil Leaks: Thicker or thinner oil could leak past gaskets or seals.
• Warning Lights: The “check engine” or “oil pressure” light coming on is a red flag.

If you notice any of these symptoms, stop driving and have the oil drained and replaced immediately.

The Long-Term Consequences of Regularly Mixing Oils

Even if you get lucky once and avoid immediate damage, regularly mixing engine oil brands can lead to cumulative issues that shorten your engine’s lifespan. Here’s what to expect over time:

1. Sludge and Deposits

When additives clash, they can form sludge—thick, tar-like deposits that clog oil passages. Sludge restricts oil flow, causing critical parts (like the camshaft or valvetrain) to overheat and wear out prematurely. In severe cases, sludge can even damage the engine’s oil pump, leading to complete failure.

2. Increased Wear and Tear

Without a consistent blend of additives, the oil’s ability to protect against friction diminishes. Metal parts rub against each other, creating microscopic scratches (called “scoring”) that worsen over time. This wear can lead to costly repairs: replacing a worn engine bearing, for example, can cost hundreds or even thousands of dollars.

3. Catalytic Converter Damage

Modern engines rely on catalytic converters to reduce emissions. Contaminants from mixed oils (like unburned fuel, metals, or phosphorus from additives) can coat the converter’s internal catalysts, rendering it ineffective. Replacing a catalytic converter is expensive—often exceeding $1,000—and may not be covered by warranty if the damage is traced to improper oil use.

4. Voided Warranty

Most vehicle manufacturers explicitly state that using non-approved oils (or mixing oils) can void your warranty. If your engine fails and the dealer determines that mixed oil was a contributing factor, you could be on the hook for thousands in repairs.

What to Do Instead of Mixing Oils

The best way to avoid the risks of mixing engine oil brands is to follow your vehicle manufacturer’s recommendations. Here’s a step-by-step guide:

1. Check Your Owner’s Manual

Your manual will specify the exact oil type (conventional, synthetic blend, full synthetic), viscosity grade (e.g., 0W-20, 5W-30), and performance standards (e.g., API SP, ILSAC GF-6) required for your engine. Never deviate from these specs unless you have explicit approval from a certified mechanic.

2. Stick to One Brand (and Type)

Once you’ve chosen an oil that meets your manual’s requirements, keep using it for all oil changes. This ensures consistency in additive chemistry and minimizes the risk of conflicts. If you’re unhappy with your current brand, finish the remaining oil in your engine before switching to a new one.

3. Plan Ahead for Oil Changes

Running low on oil should never be an excuse to mix brands. Keep a spare bottle of your preferred oil in your garage, and check your oil level monthly (more often if you drive under severe conditions, like frequent short trips or towing).

4. Consult a Professional for Emergencies

If you’re stuck and need to mix oils, call a trusted mechanic or dealership first. They can advise you on the safest options based on your engine’s design and current condition. In some cases, they may even have a sample of your recommended oil available for emergency top-offs.

Final Thoughts: When in Doubt, Don’t Mix

Engine oil is a precision product, and its effectiveness depends on the careful balance of base oils and additives. While mixing brands might seem harmless in the moment, the risks—including accelerated wear, sludge buildup, and costly repairs—far outweigh any perceived convenience. By following your manufacturer’s guidelines, sticking to one oil brand, and planning ahead for oil changes, you’ll keep your engine running smoothly and protect your investment for years to come.

Remember: Your engine is the heart of your vehicle. Treat it with the care it deserves—and that starts with using the right oil, every time.