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

At first, most of the common dilemmas of new car owners are the ideal type and brand of engine oil or whether it is safe to mix engine oil viscosities. But after 100,000 miles or about six to seven years, car owners are usually anxious if they need to high-mileage oil.

High-Mileage vs. Full Synthetic Oil

Many believe that high-mileage or old cars should use high-mileage oils rather than fully synthetic ones. Is this true? To find out, see the difference between high-mileage oil with fully synthetic oil.

First, we need to know the difference between high-mileage vs. synthetic oil.

High-Mileage vs. Full Synthetic Oil

High-Mileage vs. Full Synthetic Oil
High-Mileage vs. Full Synthetic Oil
High-mileage oil is thicker because it contains durable viscosity modifiers to keep your engine running cleaner and longer by preventing deposits from building up. While the main advantage of full synthetic oil is that it flows better in hot and cold weather, which makes it a better choice for high-performance engines.

What is High-Mileage oil?

High-mileage oil is a synthetic oil with more specialized additives specially designed for older or high-mileage engines (with more than 75,000 miles). It also contains high mileage lubricants such as:

  • conditioners
  • seal swells
  • antioxidants
  • detergents

These additives make older engines experience:

  • less wear and tear
  • prevent leaks
  • control oil breakdown


The viscosity of high-mileage oil is usually higher because it contains durable viscosity modifiers that stay in the engine for a long time. Although well-known high-mileage oil brands are available in four viscosity SAE grades 5W20, 5W30, 10W30, and 10W40.

Since it is consequently much thicker than full synthetic oil; as a result, it is suggested for older vehicles due to the reduced risk of leaks. Its thickness won’t easily leak through any small cracks that an older engine might have due to its thickness.

If the engine is more worn-out, the higher viscosity will help to slow down leakage and buy you some more time before you can take it to the repair shop.


High-mileage oils have a higher viscosity, so your engine must work harder to push the oil, resulting in decreased performance and overall oil pressure.

Summary of high-mileage oil benefits

High-mileage oil has leak-prevention additives such as seal conditioners that help stop minor leaks and seepage.

As a result, leaks are prevented, and these seal conditioners also aid in restoring the seals to their initial size and shape. Also, high-mileage oils contain more detergents than conventional oils, which aid in reducing and getting rid of sludge in the engine.

Take a look at a test conducted by Mobil 1 to prove the effectiveness of Mobil 1 Extended Performance High Mileage oil:

top deck befoe and after Mobil high mileage test
Read more: Top 5 Motor Oil Brands With High Amount of Zinc?

What is Full Synthetic Oil?

Fully synthetic oil is the purest type of synthetic oil which comes from crude oil, like conventional oil, but is artificially altered and processed with man-made chemical compounds.

Fully synthetic oil is refined, distilled, and purified to the degree that makes it possible to tailor specific oil molecules to meet the requirements of modern car engines. Because of this, it comes with a lot of benefits, such as:

  • improve mileage and fuel economy
  • better engine protection compared to mineral oil
  • less friction and drags on the engine
  • better performance both in summer and winter

The presence of various chemical additives, in addition to the refined oil base, is one of the distinctive features of full synthetic oils, supporting excellent engine performance.


Fully synthetic oils have a considerably lower viscosity than high-mileage oil, making them flow faster than high-mileage oils. As a result, fully synthetic oil flows more easily inside the engine, reducing pressure on the internal parts and lowering wear and engine damage.


Due to their low density and improved flow, synthetic oils perform better than high-mileage oils. But if protecting an old engine is to be considered, high-mileage oil performs better.

Summary of fully synthetic oil benefits

Full synthetic oils deteriorate more slowly than other engine oils, which makes them a more durable alternative to conventional oils.

Oil decomposition changes the chemical makeup of the substance, which can cause contamination. Therefore, it is necessary to replace the oil before it deteriorates, which means additional consumption of oil but not a problem with full synthetic oil.

Also, extreme temperatures can be sustained by full synthetic oil, and its flow rates are unaffected by cold temperatures, which can prevent the production of hot oil, which harms the durability and performance of the engine.

Read more: What Happens if You Drive a Car Without Oil? Expect Critical Damage

When to use high-mileage oil?

For older vehicles, high-mileage oils are advised to avoid engine leaks and minimize oil leakage. However, the 75,000-mile mark is not rigid, and the choice will largely be influenced by the state of the car’s engine.

If an old vehicle has leaks or burning oil, typically detected as blue exhaust smoke, the high-mileage oil should be used. Though, if the leak is relatively small, it might not be worth it to shell out more for high-mileage oil, which is very expensive. 

When to use fully synthetic oil?

For modern cars with high mileage, full synthetic oils are advised because of their low viscosity, which has no detrimental effects on engine performance and can prevent further leaks.

However, after using fully synthetic oil, you might notice leakage in some circumstances, but this is only due to the imbalance in the engine seals. Oil production technology is progressing, and these instances are decreasing.


You are not alone who finds it difficult to decide whether to use full synthetic oil or high mileage oil. If you have a new car or a high-mileage engine, using full synthetic oil benefits engine performance and wear-and-tear reduction capabilities.

High-mileage oil is an option if you are worried about reducing the possibility of wear and tear on a high-mileage engine, and this will help the engine last longer, but bear in mind that this isn’t a long-term fix.

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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