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How Long Can Synthetic Oil Sit in an Engine? What You Need To Know

mechanic pouring synthetic oil into car engine

Synthetic oil is a fascinating substance with millions of molecules that are nearly identical in shape and size. This uniformity is the main reason synthetic oil is better for your engine, and you can go longer between oil changes. However, synthetic oil doesn’t stay in good condition forever once it’s in your machine.

If your car, motorcycle, or other gas-powered engine sits idle, the synthetic oil inside will slowly deteriorate. How long can synthetic oil sit in an engine before you should replace it? Most experts agree that 2 years is the maximum. However, that can be slightly longer or shorter, depending on where your vehicle has been stored.

Now that you know that 2 years is more or less the maximum time synthetic oil should sit in an engine, there’s a good chance you have more questions. How long can synthetic oil be stored, for example, and can you use engine oil that’s 10 years old? If you’d like the answer to these questions and several others, we’ve got them below, plus advice and tips about storing synthetic oil. Read on to discover them all and ensure the oil in your engine is performing at its very best!

car and road divider

How Long Does Synthetic Oil Last?

One question many consumers ask about synthetic oil is how long it lasts. As with many products, the answer depends on the circumstances under which the oil is being used. Also, it depends on the type of synthetic oil being used. Typically, synthetic oil lasts for 6 to 12 months.

Mileage-wise, synthetic oil lasts between 7,500 and 10,000 miles. Of those two criteria, the amount of time the oil lasts is the most flexible. For example, if you have a classic car that you only drive on Sundays, the synthetic oil in the engine may last 2 years. However, if you drive hundreds of miles weekly, your synthetic oil might need to be changed in 2 or 3 months.

pouring oil to car engine
Image Credit: Africa Studio, Shutterstock

Does Synthetic Oil Last Longer than Conventional Oil?

One of the reasons synthetic oil has become more popular than conventional oil is that it lasts significantly longer. While conventional oil should be changed every 5,000 to 6,000 miles, synthetic oil, as we’ve seen, can last up to 10,000 miles, about 40% longer. In Europe, Castrol markets a synthetic oil that purportedly lasts upwards of 20,000 miles!

The American Automobile Association, better known to millions as “Triple-A,” has performed various tests comparing conventional types to synthetic oil and found that synthetic outperforms conventional in almost every way. For this reason, your engine can run on synthetic oil for significantly more miles before it needs to be changed.

How Long Can Synthetic Oil be Stored?

Experts agree that synthetic oil in its original container can last 7 to 8 years before it “goes bad.” However, that changes if the oil has been poured out of its original container and stored elsewhere, as oxidation can and will occur.

Why Does Synthetic Oil Last Longer than Conventional Oil?

Decades ago, aircraft engineers noticed a significant problem with conventional motor oils; they break down (degrade) under high heat. Worse than that, as they degrade, conventional oils oxidize and create sludge, a chemical residue that’s harmful to engines.

Synthetic oil removes the chemicals in conventional oil that breaks down at high temps, eliminating most of the oxidation and preventing sludge from being produced. More than that, synthetic oil doesn’t get thick at low temperatures like conventional oil, so it flows easier through your engine even when it’s freezing and thus protects your engine faster.

Why Does Synthetic Oil Cost More Than Conventional Oil?

One of the biggest questions about synthetic oil is why it costs so much more than conventional oil. The reason is simple; synthetic oil costs nearly four times as much to make as traditional oil. The refining process is highly complicated and takes longer.

Of course, when you consider that your engine can run almost twice as long with synthetic than with conventional oil, the overall savings from using synthetic oil increases considerably.

What’s the Difference Between Synthetic Oil and a Synthetic Blend

As the name suggests, a synthetic oil blend combines synthetic and conventional oil. Since both oils are fully compatible, manufacturers created a blend to provide the benefits of synthetic oil with the lower cost of conventional oil. That’s why synthetic blends cost less. However, keep in mind that you’ll need to change blended synthetic oil more frequently than straight-up synthetic oil, which will reduce your overall cost savings.

Is Synthetic Oil a Better Choice Under Normal Operating Conditions?

While there’s no doubt synthetic oil offers several benefits over conventional oil, the extra cost for some is a big drawback. Even considering that you can run your engine longer on synthetic oil, it still works out to be more expensive in most cases. Many consumers ask if synthetic oil is better under normal operating conditions.

While it probably is slightly better, synthetic oil won’t make a huge difference for most vehicles. However, synthetic oil is probably a better choice if you live in an area where extreme temperatures are common, especially extreme heat.

Do Synthetic Oils Break Down?

Everything breaks down with time, but synthetic oil lasts significantly longer than conventional oil. That’s especially true in engines that aren’t used at a high level or under extreme circumstances. In short, they break down much more slowly, which is why they need to be changed less frequently.

If you Drive Less, Can You Change your Synthetic Oil Less Frequently?

This question is one that, on its surface, seems like a no-brainer. You would assume that the less you drive, the longer your synthetic oil would last and the less frequently you need to change it.

However, if you drive infrequently, condensation that builds up in synthetic oil won’t burn off. This situation can be problematic for your engine and cause the need for more frequent oil changes. A good suggestion would be to ask your mechanic their opinion based on your driving habits.

woman driving at night
Image Credit: Daniel Jedzura, Shutterstock

What Are the Disadvantages of Synthetic Oil?

As we’ve seen, synthetic oil has several distinct advantages over conventional oil. It isn’t perfect, though, and does have several minor disadvantages.

Higher Cost

As we’ve already discussed, synthetic oil comes with a price that’s one to two times higher than conventional oil because of the more involved manufacturing process.

Additive Precipitation

Synthetic oils contain additives that can stratify or separate when your engine is exposed to cold temperatures. Under extreme circumstances, this separation can be complete, meaning that the additive and the oil will no longer be mixed.

Lower Fuel Economy at Highway Speeds

Due to a combination of factors that only a chemical engineer would completely understand, synthetic oil gets slightly lower fuel economy when driving on the highway.

How Is Synthetic Oil Made?

We’ve discussed how synthetic oil costs more due to the more involved manufacturing process. That process starts in a laboratory where each synthetic oil manufacturer has a team of highly skilled chemical engineers hard at work.

Once the engineers come up with their formula (which is proprietary and thus not publicly available), the task of refining synthetic oil is then moved to the refinery. There, raw oil (and, in some cases, natural gas) is distilled and purified. The refining process for synthetic oils is more complex than conventional oil and thus removes more impurities.

Once the impurities are removed, synthetic oil goes through several additional processes that conventional oil doesn’t. The reason is that, as we mentioned at the beginning of this article, the molecules in synthetic oil need to be almost identical in shape and size.

Think of it like this: conventional oil molecules are similar to a handful of rocks you might find in your yard. They are all different in shape and size and have many impurities. Synthetic oil molecules are similar to a bag of marbles in that they’re all practically identical in size and shape and have few impurities.

car and road divider

Final Thoughts

How long can synthetic oil sit in an engine? Typically, you won’t want to leave synthetic oil in an engine longer than 2 years, no matter the driving or running circumstances it’s exposed to. Engines used in heavy or extreme conditions should have their synthetic oil changed more frequently, and vice versa.

Synthetic oil goes through a longer and more advanced refining process, which helps it to last significantly longer than conventional oil. That process is also why synthetic oil costs more than conventional oil. In most cases, synthetic oil will last longer than traditional oil and can sit in an engine much longer without fear of engine troubles.

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Featured Image Credit: New Africa, Shutterstock


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