Understanding the Implications of Mixing Different Oil Weights in Your Vehicle

Mixing different oil weights in your vehicle can have significant implications for its performance and longevity. This article explores the potential effects, benefits, and risks of combining various oil viscosities, providing insights to help you make informed decisions about your vehicle’s maintenance.

What Are Oil Weights?

Oil weight, or viscosity, refers to the thickness or thinness of the oil. It determines how effectively the oil flows through the engine. The Society of Automotive Engineers (SAE) classifies oil weights, such as 5W-30 or 10W-40, where the first number indicates the oil’s flow at cold temperatures, and the second number represents its flow at high temperatures.

Why People Mix Oil Weights

Vehicle owners might mix oil weights for various reasons, including:

  • Emergency top-ups when the recommended oil is not available.
  • Belief that mixing can enhance engine performance or protection.
  • Attempting to adjust the oil’s overall viscosity for weather changes.

Potential Benefits of Mixing Oil Weights

While not generally recommended, there are a few potential benefits to mixing oil weights, such as:

  • Improved Lubrication: In certain cases, mixing oils can create a custom viscosity that might offer better protection under specific conditions.
  • Emergency Solutions: Mixing oils can be a temporary solution if the correct oil is not available, potentially preventing engine damage due to low oil levels.

Risks and Drawbacks

Despite the potential benefits, mixing oil weights carries several risks:

  • Viscosity Breakdown: Mixed oils may not provide a stable viscosity, leading to reduced lubrication and increased wear.
  • Chemical Incompatibility: Different oil formulations might react negatively, affecting engine performance and longevity.
  • Warranty Issues: Using a non-recommended oil mixture could void your vehicle’s warranty.

Expert Recommendations

Most automotive experts advise against mixing oil weights. Instead, they recommend following the vehicle manufacturer’s guidelines for oil viscosity and type. If you must mix, it’s crucial to:

  • Stay within the same brand and formulation to minimize compatibility issues.
  • Limit the practice to emergency situations and change the oil as soon as possible.

Understanding Oil Viscosity Ratings

The table below outlines common SAE viscosity ratings and their typical applications:

SAE Viscosity Cold Weather Performance Hot Weather Performance
5W-30 Excellent Good
10W-40 Good Excellent
20W-50 Poor Excellent

FAQs on Mixing Oil Weights

Can mixing oil weights damage my engine?

Mixing oil weights can potentially lead to engine damage due to viscosity breakdown and chemical incompatibility, especially if done frequently or without proper understanding.

Is it better to mix oils or use a single incorrect weight?

Using a single incorrect weight might be less harmful than mixing, as the oil’s properties are consistent. However, always aim to use the manufacturer-recommended oil weight.

How often should I change mixed oil?

If you’ve mixed oils in an emergency, it’s best to change the oil as soon as possible, ideally within a few hundred miles, to ensure your engine is protected.


Mixing different oil weights in your vehicle is generally not recommended due to the potential risks and drawbacks. While there might be temporary benefits in emergency situations, the best practice is to use the manufacturer-recommended oil weight and type. Regular maintenance and adherence to your vehicle’s specifications are crucial for ensuring optimal performance and longevity.