Is Synthetic Oil Better for High-Mileage Car? Benefits of Synthetic High-Mileage Oil

One of the ways to give your vehicle’s engine a more extended life is by using synthetic oil for high-mileage cars. Though it can be more pricey than synthetic blends and conventional mineral oil, it has more added benefits for all types of engines.

Synthetic Oil Better for High-Mileage Car

We’ve already compared synthetic and semi-synthetic oil — some swear by synthetic, and some say it’s not worth the extra price tag. You’ve probably heard the pros and cons of both camps, but which one is true? 

Which oil works best for your car — particularly if you have a high mileage car? Let’s discuss this below.

Is synthetic oil better for high-mileage cars?

Yes. Synthetic oils are better than conventional oils for high-mileage vehicles because they have better cold flow and perform better at higher temperatures. Alternately, it's best to use high mileage oils that can rejuvenate degraded seals and break up residual sludge in vehicles with more than 75,000 miles.

What is high mileage synthetic oil?

High mileage synthetic oil is a type of motor oil designed to be used in older vehicles or those over 75,000 miles. Although a full synthetic is safe for high-mileage cars, we don’t advise fully synthetic oil for some older or classic cars (cars built before the 1990s).

The synthetic oil tends to clean away some of the gunk and grime that might be what’s helping your seals stay sealed.

Instead of using fully synthetic oil, we recommend using the high mileage synthetic oil instead. As mentioned above, they are designed for older cars, and it has many benefits, including:

  1. Better protection
  2. Reduced engine wear
  3. Rejuvenate degraded seals
  4. Improve fuel efficiency

Better protection against sludge and engine wear

Sludge is a mixture of metal particles and other deposits that can build up inside your engine. This results in:

  • poor performance
  • rough idling
  • decreased fuel economy

Synthetic oil offers better protection against sludge than conventional oil because it contains additives that help keep your engine clean.

Reduced engine wear

The synthetic molecules in high mileage oils are more uniform in shape with fewer impurities. They can get into areas where conventional oils cannot reach and lubricate them more thoroughly. 

In effect, it reduces friction and helps extend the life of internal engine parts such as pistons and camshafts.

Rejuvenate degraded seals

High mileage oil also contains more detergents than conventional oils do. These detergents can help clean out deposits inside your engine and restore degraded seals — which may be why some say they notice fewer car problems after switching to high mileage oil.

One of the most popular high mileage oil is the Castrol GTX High-Mileage. Specially formulated motor oil with unique additives to protect engines. Aside from the benefits, it reduces phosphorous to protect your emission system.

As we all know, high zinc and phosphorus motor oil may compromise the catalytic converter.

Still, the best way to protect older engines found in high-mileage cars is to use the recommended oil at the recommended intervals. 

The manufacturer knows what is best for their engines and has gone through extensive testing to determine the best viscosity and additives for each engine type.

Frequently Asked Questions

Is it true that you cannot switch back once you switch to synthetic oil?

No, it is a myth. In reality, aside from conventional (mineral-based) oil and full synthetic oil, synthetic blends are also available in the market. Synthetic blends combine synthetic and conventional oils meaning synthetic and conventional oils are safe to interchange.

Although mixing oil types and viscosities (e.g., 5W20 and 10W30) are generally safe, we still strongly advise using the same oil for top-ups if necessary. This is to get the most protection from the oil you have chosen,  

Read more: Can You Mix Oil Viscosities? 5 Important Things You Should Know 

Will using synthetic oil result in fewer regular oil changes?

Yes. The main reason why many auto manufacturers recommend synthetic over conventional motor oils is that they’re designed to last longer between changes. It doesn’t break down like regular oil, so you can go longer between oil changes. Cars that need full-synthetic motor oil might go as far as 15,000 miles between services.

Still, you should follow the change interval recommendations on the car’s manual. It is important to change your oil regularly, whether synthetic or not.

Read more: Can You Add Oil While The Engine is Hot? Top 6 Questions [Solved]

Will using synthetic oil cause leaks? 

No, synthetic oil will not cause leaks or harm engine seals. On the other hand, if your engine has a leak or a damaged seal, it may help you find it. Due to its molecular homogeneity and low viscosity, synthetic oil has a higher chance of flowing through narrow holes. That is advantageous when it comes to pushing its way through the high precision of a modern engine. 

However, there are isolated cases for cars made before the 1990s where full synthetic oil clean away some of the gunk and grime that might be what’s helping your seals stay sealed.

What are synthetic oils made from? 

Synthetic oil is a man-made alternative to petroleum-based motor oil. It’s produced by chemically modifying base materials, such as petroleum components. The synthesis process for creating synthetic oil varies among producers and is considered a trade secret. The result is a lubricant that can provide better performance and protection.

Is synthetic oil better than conventional oil for your engine? 

Yes, synthetic oil is better than conventional oil for your engine. Even though conventional oil offers sufficient lubrication, synthetic oil outperforms it in terms of overall engine protection and performance. Base oils used to make synthetic oils are of better quality than typical, less refined base oils. 

These higher-quality base oils make synthetic oil more chemically stable, less prone to oxidizing and acidifying, and more difficult to break down and lose desirable properties.

Can I use synthetic oil if my car doesn’t require it? 

Yes, you can use synthetic oil even if your car doesn’t require it. Some engines demand synthetic oil, while the majority do not. Although the car manufacturer may not recommend it, synthetic motor oil can be used instead of conventional oil.


The last thing you want is to ruin your car because of the type of oil you choose. If you have a high mileage car (75k miles +), then synthetic oil may be a wise choice for your car. The benefits far outweigh the increased cost of synthetic oil.

More than that, synthetic high-mileage oil is better than mineral oil because it can hold up in the engine better, and thus, you will be able to save more money on oil changes. You may find oil filter changes less often with synthetic high mileage oils.

Elevate your car maintenance game by reading our comprehensive guide on engine oil. Learn everything from the basics of oil types, to how to properly maintain and change it. Keep your vehicle running at its best, read our guide now!

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