Switching To Synthetic Oil After 100k Miles [The Myth & Truth]

Synthetic oil prevents sludge buildup and protects your engine by reducing frictions. Plus, it improves fuel efficiency. Considering these benefits, you decide to switch to synthetic oil in your high mileage car.

But the synthetic oil myth ‘ switching to a synthetic oil causes a leak’ stops you from doing that as your car reaches the 100k miles mark. 

The reality is not like that at all. Today, we will explore whether switching to synthetic oil after 100k miles is safe or not for your car. So, keep reading between the lines to discover the truth behind the myth. 

Will Synthetic Oil Cause Leaks?

Switching to synthetic oil will cause a leak. Generally, it’s a synthetic motor oil myth. In reality, it doesn’t cause any leaks. This oil type is indeed thinner than the conventional engine oil. That’s why it flows with ease. 

If there is a spot where there is a possibility to leak, the synthetic oil then will leak more than your traditional engine oil. However, synthetic oil causes no leak.

Note: You can also read if high mileage oil causes leaks or not.

Is Switching To Synthetic Oil After 100k Miles Safe?

There is a lot of myth or controversy about switching to synthetic oil in higher mileage vehicles. So, it’s a bit typical to give a precise answer to this question. How you maintain the car and what type of oil you used before will determine whether you can switch to synthetic oil or not after 100k miles. 

You can safely switch to synthetic oil if you maintain your car properly and it doesn’t leak oil. 100k miles will never be too late to try synthetic oil to extend the life of your engine. 

Do an oil change at 3000-5000 miles if you make the switch to full synthetic. It is because there are surely some extra remnants or internal deposits. And the frequent oil change will purge out or clean out the old oil buildups. Then, you can go 7500-10000 miles between oil changes depending on your car. 

The type of oil you have been using when you reach the 100k mark will also determine if you go with the synthetic or not. We see no reason that switching to synthetic will be a problem if you previously maintained your vehicle with quality detergent oil. 

Lastly, choose the right viscosity of the oil if you make the switch. We recommend you go with a better range multi-grade. If your car was recommended for 10W-30, you can use 5W-30 or 0W-30 to maximize lubrication & performance in cold weather. 

When To Not Switch To Synthetic Oil After 100K Miles?

You can’t switch to full synthetic oil if you don’t use it from the beginning. Therefore, you shouldn’t use synthetic oil if your engine uses rope-type oil seals and cork gaskets. 

Now, let’s break down why you shouldn’t switch to synthetic oil if you have been using regular oil from the beginning. If you go with the synthetic oil, it will leak through all those spots regular oil was protecting for you as this oil type is thin. 

Similarly, you will see a problem switching oils if your engine has seen years of non-detergent-type oil in it. If you switch to a detergent oil like synthetic, it will clean all the deposits out and start leaking. 

Lastly, full synthetic oil will just leak past the older type of seals and gaskets. Now, you may ask- Can’t I switch to synthetic oil if my car has those issues?

Yes, you can. But don’t directly switch to full synthetic oil. First, go with the synthetic blend. Then, shift from the synthetic blend to the full one gradually. However, it is better to stay with the oil you have been using for years to keep the engine safe. 


Is it bad to switch from conventional oil to synthetic?

It is okay when it comes to switching from conventional oil to synthetic. The simple switching will never affect your traveling or engine. 

Will synthetic oil hurt older engines?

No, synthetic oil will not hurt older engines as this oil type now has no chemical compounds in it. Indeed, it’s safe and improves the old engine performance. 

Can you use synthetic oil after using regular oil?

Yes, you can use synthetic oil after using conventional oil. However, we recommend you contact your car manufacturer regarding this. 

What are the disadvantages of synthetic oil?

Synthetic oils are expensive and offer lower fuel economy at highway speed. 

How Long Will synthetic oil last?

It will last 7500-10000 miles on average between oil change intervals.


Undoubtedly, you can switch to synthetic oil after 100k miles if you maintained your car adequately throughout the years. On the other hand, you should avoid synthetic oil if your car leaks.

Never switch to directly full synthetic oil if you have used conventional oil from the beginning. If you are still confused, read this article again from top to bottom. 

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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