Can You Mix Engine Oils

Mixing engine oils is one of the most common questions car owners, DIY mechanics, and even apprentice technicians have, yet it remains one of the least properly explained topics. Every vehicle, whether it runs a small-displacement petrol engine or a high-torque diesel powerplant, relies heavily on lubrication engineering. Engine oil isn’t just a slippery liquid; it’s a carefully engineered formulation that contains base oils, viscosity modifiers, anti-wear additives, dispersants, detergents, friction modifiers, antioxidants, corrosion inhibitors, and more. Because of this, mixing oils is more than simply pouring one brand into another. It’s a question that involves chemistry, mechanical engineering, tribology, thermal management, and Australian driving conditions.

Understanding Engine Oil: The Fundamentals Behind the Fluid

Before answering whether you can mix engine oils, we must understand what engine oil really is.

1. Base Oils

Engine oils are built from different base oil groups:

• Group I – Mineral oil, solvent refined

• Group II – Hydrotreated mineral oils

• Group III – Hydrocracked synthetic (used in many “synthetic” oils in Australia)

• Group IV – PAO (polyalphaolefin true synthetic)

• Group V – Ester-based oils (high-performance, racing, industrial-grade lubricants)

When you mix oils from different groups, the base oil chemistry must remain compatible. Otherwise, you may experience:

• Foaming

• Shear instability

• Reduced lubrication film strength

• Accelerated oxidation

Modern oils are generally mixable, but performance may be compromised.

2. Additive Packages

Additives make up 15–30% of the oil formulation and include:

• Detergents (neutralise acids)

• Dispersants (suspend contaminants)

• Anti-wear agents (ZDDP – zinc dialkyldithiophosphate)

• Anti-oxidants

• Friction modifiers

• Viscosity Index Improvers

• Anti-foam additives

• Corrosion inhibitors

If you mix two oils with different additive packages, they may not work harmoniously.

3. Viscosity Grades

Oil viscosity is defined by the SAE J300 standard, e.g.:

• 5W-30

• 10W-40

• 0W-20

Mixing oils with different viscosities creates a blended viscosity that is not equivalent to either original grade.

Example: Mixing 5W-30 with 10W-40 may result in something approximating 7W-35, but with unpredictable thermal and shear behaviour.

4. Industry Standards

Engine oils carry:

• API ratings (e.g., SN, SP, CK-4)

• ACEA classifications (A3/B4, C2, C3 etc.)

• ILSAC standards

• Manufacturer approvals (e.g., VW 504.00/507.00, MB 229.5, GM Dexos, Ford WSS-M2C913-D)

Mixing oils that do not meet the same standard may reduce the vehicle’s compliance with required specifications.

Now… Can You Mix Engine Oils? The Real Answer

Short answer:

Yes, you can mix engine oils in an emergency, but it is not recommended unless absolutely necessary.

Long answer:

You can mix oils as modern formulations are designed to be compatible with each other, so they don’t gel, react, or solidify. However, mixing oils will not necessarily give you the protection, viscosity stability, or manufacturer-specified performance level that the engine was designed for.

Let’s break this down technically.

When Mixing Engine Oils is Acceptable (Emergency Situations)

1. Low oil level and no option for the exact oil

If the oil level drops below the minimum on the dipstick, running the engine could lead to:

• Oil starvation

• Bearing failure

• Camshaft wear

• Turbocharger damage

• Excessive heat due to reduced lubrication film

In such cases, mixing a different oil grade or brand is still better than running low oil.

2. Topping up with a different viscosity

If your engine requires 5W-30 and you only have 10W-40, topping up is acceptable temporarily. It will not cause immediate damage, but it will change the viscosity characteristics.

3. Emergency roadside conditions

Australian conditions (heat, long-distance drives, dusty environments) require proper lubrication. If the engine is consuming oil or leaking, adding any compatible oil is safer than running dry.

When Mixing Engine Oils is NOT Recommended

1. Mixing synthetic with mineral oils

While technically compatible, it may:

• Dilute the synthetic oil’s benefits

• Reduce oxidation stability

• Affect cold-start lubrication

• Reduce high-temperature film strength

2. Mixing oils with different manufacturer approvals

Example:

• VW 504.00

• MB 229.52

• Dexos 1 Gen 3

OEM approvals involve strict performance testing. Mixing oils voids the approval.

3. Mixing oils with drastically different viscosities

Mixing 0W-20 with 15W-50 creates:

• Unstable viscosity index

• Poor cold-start performance

• Reduced protection at operating temperature

• Higher likelihood of sludge formation

4. Mixing diesel oil with petrol-only formulations

Diesel oils have higher detergent content. Petrol engines may experience:

• Spark plug fouling

• Increased deposits

• Catalytic converter stress

5. Engines under warranty

Manufacturers may reject claims if oil grades or specifications do not match required standards.

The Science Behind Why Mixing Oils Is Not Ideal

Additive Clash

This happens when two inhibitors or detergents contradict each other.

Example:

• Oil A uses molybdenum as a friction modifier

• Oil B uses titanium instead

Combined, they may reduce lubrication efficiency instead of enhancing it.

Shear Stability

Viscosity Index Improvers break down over time. Mixing oils with different shear stability characteristics can lead to rapid thinning under high-load conditions.

Thermal Breakdown

Australian climate can exceed 40°C in many regions. Mixing oils with different heat-resistance levels results in:

• Loss of viscosity

• Oxidation

• Formation of sludge and varnish

Detergent Load Mismatch

Diesel oils contain high detergent levels. Petrol engines may experience washdown of lubricating films.

Industrial Perspective: Why Oil Blending is a Specialty

Industrial lubrication engineers blend oils using strict laboratory practices:

• Base oil compatibility testing

• Additive solubility analysis

• Foam tendency testing

• Oxidation stability tests

• Shear breakdown measurements

• High-temperature high-shear (HTHS) viscosity testing

Randomly mixing oils in your garage is nothing like engineered blending.

Australian Driving Conditions: Why It Matters

Australia’s environment is harsh:

• High heat

• Long-distance highway driving

• Dusty rural roads

• Stop-and-go city traffic

• Heavy towing loads

Because of this, oil quality must remain optimal. Mixing oils compromises the consistency required for local conditions.

What Happens Inside the Engine if You Mix Oils?

Best-case scenario

• Oil functions normally

• No immediate issues

• Engine survives without damage

Average-case scenario

• Slight reduction in performance

• Increased oil consumption

• Reduced fuel economy

• Faster oxidation

• Shortened oil change interval

Worst-case scenario

• Varnish buildup

• Sludge formation

• Turbo failure

• Timing chain wear

• Loss of viscosity

• Reduced protection under heavy load

How to Correctly Handle Mixed Oil

If you have mixed oils accidentally or out of necessity:

1. Schedule an earlier oil change

Instead of 10,000 km, change at:

• 3,000–5,000 km

2. Replace the oil filter

Old oil may carry contaminants.

3. Use the correct oil next time

Follow:

• Manufacturer specifications

• Viscosity requirements

• ACEA/API ratings

4. Monitor your engine for symptoms

Look for:

• Noises

• Excess consumption

• Rough idle

• Oil leaks

Practical Examples

Example 1: Mixed 5W-30 Synthetic with 10W-40 Semi-Synthetic

Effect:

• Viscosity slightly higher

• Cold start lubrication slightly poorer

• Acceptable short-term

• Replace oil early

Example 2: Mixed Diesel Oil 15W-40 with Petrol Engine 5W-30

Effect:

• High detergent load → possible deposits

• Slight MPG drop

• Not ideal long-term

Example 3: Mixed Full Synthetic 0W-20 with Mineral 20W-50

Effect:

• Highly unstable viscosity

• Possible sludge

• Cold start issues

Advantages of Not Mixing Oils

1. Maximum engine protection

2. Improved fuel economy

3. Consistent lubrication film

4. Better high-temperature resistance

5. Longer engine life

6. Warranty compliance

7. Better performance for Australian climate

FAQs

1. Is mixing engine oils harmful?

Not immediately, but it may reduce performance and shorten oil life.

2. Can I mix synthetic with semi-synthetic?

Yes in emergencies, but it reduces the benefits of full synthetic oil.

3. Will mixing void my warranty?

If the oil no longer meets OEM specifications, yes, manufacturers may reject claims.

4. Can mixing oils cause sludge?

Yes if the additive packages clash or viscosity becomes unstable.