While generally not recommended, mixing motor oil weights may be permissible in limited circumstances, but certain types of oil should never be combined.
Understanding Motor Oil Weights
Motor oil "weight" refers to its viscosity, or how easily it flows at different temperatures. For example, a 5W-30 oil indicates its cold-weather viscosity (5W) and its high-temperature viscosity (30). Using the correct oil weight is crucial for your engine's lubrication and longevity.
When Mixing Motor Oil Weights (Viscosities) is Permissible (with Caution)
According to expert advice, it's typically not a good idea to mix oils, though you may be able to get away with it if the grades are fairly similar. This means a minor adjustment, such as using a slightly different viscosity, might be acceptable as a temporary measure or in an emergency.
Scenarios where a slight mix might be tolerable (though not ideal):
- Minor Top-Off: If your oil level is critically low and you only have a slightly different, but similar, weight on hand (e.g., adding 10W-30 when 5W-30 is recommended).
- Similar Grades: As the reference states, you might "get away with it if the grades are fairly similar (such as using a 30 grade when a 40 grade is recommended)." This suggests a small difference in the high-temperature viscosity might not cause immediate catastrophic failure, but it's still a compromise.
Important Considerations for Mixing Similar Grades:
- Temporary Solution Only: Any mixing of different weights should be considered a temporary fix until you can perform a full oil change with the correct, manufacturer-recommended oil.
- Minimal Quantity: The less you mix, the better. A small amount of a slightly different viscosity for a top-off is less impactful than a full blend.
- Consult Your Owner's Manual: Always prioritize the oil specifications listed in your vehicle's owner's manual. It is the definitive guide for your specific engine.
When Mixing Motor Oil is Strictly Prohibited
One critical rule from the reference is clear: You should never mix conventional oil with synthetic oil. This applies regardless of the oil weights.
| Oil Type Mixing Scenario | Recommendation |
|---|---|
| Similar Viscosity Grades | Generally Not Recommended, but May be okay for small, temporary top-offs. (e.g., adding 30-grade to 40-grade) |
| Conventional & Synthetic | NEVER MIX. |
| Vastly Different Viscosities | NEVER MIX. (e.g., 5W-20 with 20W-50) |
Why Mixing Conventional and Synthetic Oils is a Bad Idea
Conventional and synthetic oils use different base stocks and additive packages. While modern oils are designed to be compatible to some extent, mixing them can:
- Degrade Performance: The unique properties of synthetic oil (e.g., extreme temperature stability, longer change intervals) can be compromised by the presence of conventional oil.
- Compromise Additives: The additive packages in conventional and synthetic oils are formulated differently. Mixing them can dilute or neutralize their effectiveness, leading to reduced wear protection, increased sludge formation, and overall poorer engine health over time.
- Void Warranty: In some cases, mixing oil types not approved by the manufacturer could potentially void your vehicle's engine warranty.
Best Practices for Motor Oil
To ensure your engine's longevity and optimal performance, always adhere to these best practices:
- Use Recommended Oil: Always use the motor oil weight and type (conventional, synthetic, synthetic blend) specified in your vehicle's owner's manual.
- Consistent Brand & Type: Ideally, stick to the same brand and type of oil for consistency, although this is less critical than using the correct weight and never mixing conventional and synthetic.
- Regular Oil Changes: Follow the manufacturer's recommended oil change intervals.
