Can I Mix High Mileage Oil With Regular Oil

The question of whether you can mix high mileage oil with regular oil is a common one, especially as vehicles age. The short answer is: yes, you *can* mix them, but it's essential to understand the implications. Let's delve into the details to help you make informed decisions about your engine's lubrication.

Understanding High Mileage and Regular Oils

First, let's clarify what high mileage and regular oils are and how they differ. Regular motor oils are formulated to meet the lubrication needs of engines in relatively good condition, typically with fewer miles on the odometer. These oils contain detergents, dispersants, and other additives to keep the engine clean, prevent sludge buildup, and protect against wear.

High mileage oils, on the other hand, are specifically designed for vehicles with 75,000 miles or more. They often contain a higher concentration of certain additives, such as:

Seal conditioners: These additives help to rejuvenate and swell worn seals, reducing or preventing oil leaks common in older engines. Imagine the valve cover gaskets in an older BMW 3 Series (E46) – they're notorious for leaking as they age. Seal conditioners can help slow or stop those leaks.
Viscosity improvers: As engines age, the clearances between moving parts can increase, leading to lower oil pressure. Viscosity improvers help maintain adequate oil film thickness at operating temperatures, even with increased clearances. This is particularly crucial in older engines like a classic Ford Mustang with a 289 V8, where tight tolerances weren't always a priority from the factory.
Anti-wear additives: These additives, like zinc dialkyldithiophosphate (ZDDP), provide extra protection against wear, especially during startup when oil pressure is low. ZDDP is particularly important for older engines with flat-tappet camshafts, such as those found in many classic American cars.
Detergents and dispersants: High mileage oils usually have enhanced detergents and dispersants to combat the increased sludge and deposits that can accumulate in older engines.

Consider a Honda Accord with over 150,000 miles. Over time, seals can harden and shrink, leading to oil consumption. Switching to a high mileage oil can help recondition those seals and reduce oil loss. Or imagine a Toyota Camry with similar mileage; the engine might start exhibiting slight ticking noises at startup. A high mileage oil's anti-wear additives could help minimize that wear.

The Impact of Mixing Oils

When you mix high mileage oil with regular oil, you essentially dilute the concentration of the additives in the high mileage oil. Here's a breakdown of the potential effects:

Reduced effectiveness of seal conditioners: Mixing reduces the amount of seal conditioners available to rejuvenate seals. This could mean that oil leaks might not be addressed as effectively as they would with a full fill of high mileage oil.
Compromised viscosity protection: The viscosity improvers in the high mileage oil will be diluted, potentially reducing their ability to maintain adequate oil film thickness, especially at high temperatures. If you're driving a high-performance vehicle like an older Porsche 911, maintaining proper oil viscosity is crucial for engine longevity.
Lowered anti-wear protection: Diluting the anti-wear additives in the high mileage oil means the engine might not receive the same level of protection against wear, especially during cold starts.
Potentially altered detergent and dispersant balance: While both oils contain detergents and dispersants, their formulations might differ. Mixing them could theoretically affect their effectiveness in cleaning and preventing sludge buildup, though this is less of a concern than the dilution of seal conditioners and viscosity improvers.

It's important to note that mixing oils won't necessarily cause immediate catastrophic engine damage. Modern motor oils are designed to be compatible with each other. However, you're not getting the full benefit of the high mileage oil if you're diluting it with regular oil.

When Mixing Might Be Acceptable

There are situations where mixing high mileage and regular oil might be acceptable, although not ideal:

Emergency top-off: If you're on a road trip and your oil level is low, and you only have regular oil available, topping off with a quart or less is better than running the engine with insufficient oil. This is especially true for vehicles known for oil consumption, like some older Subaru models. Just remember to switch back to a full fill of high mileage oil at the next oil change.
Slightly overfilling: If you slightly overfill the engine during an oil change and need to remove a small amount of oil, you could theoretically replace it with regular oil. However, it's generally better to drain the excess oil rather than adding a different type.

Even in these situations, it's crucial to use the correct viscosity of oil. For example, if your engine requires 5W-30, ensure that both the high mileage and regular oil are 5W-30.

The Best Practices

To ensure optimal engine performance and longevity, follow these best practices:

Use the oil type recommended by the manufacturer: Check your owner's manual for the recommended oil viscosity and specification.
If using high mileage oil, stick with it: Once you switch to high mileage oil, consistently use it at each oil change to maximize its benefits.
Change your oil regularly: Follow the manufacturer's recommended oil change interval. Shorter intervals might be beneficial for older engines or those subjected to severe driving conditions.
Address oil leaks promptly: While high mileage oil can help mitigate leaks, it's essential to address the underlying cause of the leaks to prevent further damage.

Consider Synthetic Blends

A synthetic blend high mileage oil is often a good compromise. These oils offer some of the benefits of full synthetic oil (better high and low temperature performance, improved oxidation resistance) along with the additives specific to high mileage formulations. Think of it as a 'best of both worlds' approach.

Read the Labels

Always read the oil container labels carefully. Pay attention to the oil's viscosity, API (American Petroleum Institute) rating, and any specific claims made by the manufacturer (e.g., "suitable for turbocharged engines").

Practical Takeaways

Mixing high mileage and regular oil dilutes the benefits of the high mileage oil. Avoid doing it unless absolutely necessary.
If you must mix, use the correct viscosity of oil. For example, mixing 5W-30 high mileage with 5W-30 regular is better than mixing 5W-30 high mileage with 10W-40 regular.
If you've switched to high mileage oil, stick with it.
Consider a synthetic blend high mileage oil for enhanced performance.
Prioritize regular oil changes and address oil leaks promptly.

By understanding the differences between high mileage and regular oils and the potential consequences of mixing them, you can make informed decisions about your vehicle's lubrication and help ensure its long-term health and performance. Remember, a little preventative maintenance goes a long way, especially for older vehicles.