High-Mileage vs. Full Synthetic Oil: What’s the Difference?

Car engines require proper lubrication for optimal performance and fuel efficiency. But which engine oil is best for your car? Many drivers are confused about whether to use high-mileage oil or full synthetic oil.

Full synthetic oils are the preferred option for most cars. In contrast, high-mileage oils are specifically designed for older vehicles with high-mileage engines to help minimize engine wear. That said, modern full synthetic oils also offer the same benefits as high-mileage oil minus its drawbacks.

In this article, I’ve put together an in-depth comparison between high-mileage oil and full synthetic oil. You’ll learn exactly how each oil type is different, which one you should use for your car, and why.

Differences Between High-Mileage and Full Synthetic Oil

Both full synthetic oil and high-mileage oil are a replacement to the cheaper conventional oils that are typically used in car engines.

Full synthetic oils offer better engine wear protection, keep your engine cleaner, and have better flow at both higher and lower temperatures.

On the other hand, high-mileage oils have many benefits of full synthetic oils but are optimized for vehicles with over 75,000 miles. This is because cars become susceptible to wear and damage after such high mileage.

That’s why industry experts suggest that older engines require lower flowing oils like high-mileage oil — to minimize this wear and tear on the engine parts.

Let’s discuss high-mileage and full synthetic oil in more detail.

What Are High-Mileage Oils?

High-mileage oils are designed for motor engines with more than 75,000 miles on them. It comes with various additives that help minimize engine wear while extending engine life.

As your car’s engine gets older, its internal seals and gaskets shrink and weaken.

For instance, the valve guide seals will naturally wear out over your car’s lifetime, resulting in oil leakage into the engine’s combustion chambers.

This leakage can result in smoke formation. The oil will seep into the chamber and come out as blue exhaust smoke. This may negatively impact your car’s oil level.

Now, high-mileage oil can prevent this from happening thanks to its two main features:

  • It has additives to prevent leaks
  • It has a higher viscosity

Let me elaborate.

High-Mileage Oil Has Additives to Prevent Leaks

Some high-mileage oils contain seal conditioners that help reduce seepage and minor leaks. These seal conditioners also help restore the seals to their original size and shape, thereby preventing leaks from happening.

Apart from seal conditioners, these high-mileage oils also feature more detergents than traditional oils. This helps to minimize and eliminate sludge in the engine.

Other than this, many high-mileage oil manufacturers include other additives to help reduce wear and tear for a longer-lasting engine.

High-Mileage Oil Has a Higher Viscosity

High-mileage oil is a higher viscosity oil. Accordingly, it’s much thicker than full synthetic oils, and as such, it’s recommended for older vehicles because of the lower risk of leaks.

Since it’s thicker, it won’t easily leak through the fine cracks that might be present on older engines. Whereas, in the case of more worn-out engines, its higher viscosity will help reduce the leakage and buy you some extra time till you can take it to the mechanic.

When to Use High-Mileage Oil

Making the switch to high-mileage oil is a matter of preference. One may argue that most oil types claim to have a wide array of additives that help your engine, so the presence of additives may not necessarily help in making the decision. This draws attention to the impact marketing has on how these products are perceived.

High-mileage cars are recommended for older cars to prevent engine leaks and reduce oil leakage in older cars. However, the 75,000 miles mark isn’t written in stone, and your decision will largely depend on your vehicle’s engine condition.

You should use high-mileage oil if you have an old car and notice signs of leakage or burning oil, typically identified as blue exhaust smoke. However, if the leakage is minor, the additional cost of high-mileage oil may not be justified.

It’s also worth considering that high mileage can mean different things.

For example, with much better roads and improved vehicle technology, most cars can have a lifespan of up to 400,000 miles.

The impact of mileage on your car will depend on various factors, including your driving approach, whether or not you drive on the highway or city, and your car’s maintenance. So it’s prudent to keep this in mind when gauging your car’s engine condition.

For instance, you might need to switch to high mileage oil at 80,000 miles or even 200,000 miles.

In the end, using high-mileage oil is primarily a precaution to help limit wear and tear. Whether or not you make the switch to high-mileage oil is therefore largely subjective and a matter of preference.

That said, it’s worth pointing out that high-mileage oil can only reduce or mitigate wear and tear for so long. It’s not a permanent solution against all mechanical wear on your engine. However, it is a recommended approach if you want to extend your car’s engine life.

See also: Signs Your Car Has Reached Maximum Mileage

What Are Full Synthetic Oils?

Full synthetic oils are a low viscosity motor oil. As a result, this oil flows more quickly than conventional oils or even high-mileage oils. Accordingly, this allows your car’s engine to run more efficiently because smoother oil flow translates to interruption-free engine performance.

One of the unique aspects of synthetic oils is that they have a variety of chemical additives on top of their refined oil base. Some of the additives include castor waxes, paraffinic oils, and polyalphaolefin. These additives help to maintain optimal engine function.

Advantages of Using Full Synthetic Oils

Here’s a look at some of the main advantages of using full synthetic oils:

  • Full synthetic oils degrade slowly compared to other engine oils. While all oils break down, the rate at which this happens differs across different types of oils. When oil breaks down, its chemical composition gets altered, leading to contamination. As such, the oil needs to be replaced before it degrades. However, synthetic oils take longer to degrade, making them a more lasting option when compared to traditional oils.
  • Full synthetic oil can withstand extreme temperatures. The intense heat produced by the engine can heat the oil, which can negatively impact engine performance and durability. Also worth considering, the flow rate of full synthetic oil isn’t affected in cold weather, which also ensures stable performance.
  • Full synthetic oils are less viscous. As a result, the oil flows better inside the engine, minimizing the strain on the internal components and thus reducing wear and damage to the engine.

When to Use Full Synthetic Oils

Full synthetic oils can be used on new cars and high mileage engines. Their low viscosity doesn’t negatively impact engine health nor increase leakages.

However, in some situations, you might notice instances of leakage following the use of full synthetic oils. This might be due to a mismatch in the engine seals. Thankfully, reports of this effect are reducing as oil and car manufacturing technology progresses.

FAQs About High-Mileage and Full Synthetic Oils

Do Full Synthetic Oils Offer Better Performance Than High-Mileage Oils?

Full synthetic oils offer better performance than high-mileage oils because they’re low density and flow better. In contrast, high-mileage oils have higher viscosity which means your engine will have to work extra hard to push the oil, leading to reduced performance.

Can You Use Full Synthetic Oil in a High Mileage Engine?

You can use full synthetic oil in a high mileage engine, and it won’t damage the engine nor negatively impact performance. Previously, this wasn’t possible because of the low viscosity of full synthetic oil leading to leakage. But current progress in technology has resolved this problem.

Currently, the idea that full synthetic oils are bad for high-mileage engines is a myth.

This myth is anchored on the notion that seals in older cars are incompatible with low viscosity oils because the oil might leak through the wear in these old engines. However, this isn’t backed by the present literature.

Can You Use High-Mileage Oils in New Engines?

You shouldn’t use high-mileage oils in new car engines, and hardly any manufacturer recommends this approach. The thickness of high-mileage oil, coupled with how it can potentially impact startups and engine flow to critical engine parts, makes it an ill-advised option for newer cars.

For newer cars, you should consider full synthetic oils.


If you’re struggling with choosing between high-mileage oil and full synthetic oil for your car, you’re not alone. Due to its positive impacts on engine performance and wear-and-tear-reduction capabilities, I recommend the full synthetic oil, whether you have a new car or a high-mileage engine.

If you’re concerned about eliminating the risk of wear and tear on a high-mileage engine, then you can opt to get high-mileage oil to help extend engine life. However, keep in mind that this isn’t a permanent solution.

Related: Can You Add Oil While the Engine Is Hot?

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