What happens if you put conventional oil in a car that requires synthetic

Synthetic oil, once found mostly in high-performance cars, is being used in more mainstream vehicles.

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What happens if you use regular oil instead of synthetic?
Can I put conventional oil in a synthetic car?
Will the wrong oil damage my engine?

Take, for example, the popular Honda Civic. A Honda spokeswoman said the automaker uses a synthetic oil in the Civic for its low viscosity—meaning it flows more easily than oils with higher viscosity—which helps to improve the engine's fuel efficiency. The faster the oil flows, the quicker the engine parts can turn.

Honda is not alone, says Mel Yu, auto analyst at Consumer Reports. In the 2019 model year, about 70 percent of new cars get either fully synthetic or blended oil.

Several brands, including Honda, don't specifically require synthetics for their engines, but the low-viscosity oils that those engines need are offered only in a synthetic format, Yu says. Some brands use “synthetic blend” oil, which is a combination of conventional and synthetic oils. Blends don’t deliver the full benefits of synthetic, but they’re considerably cheaper, he adds.

On top of that, many oil-change outlets also offer synthetic oil as an alternative to conventional engine oil.

According to Will Hixson, spokesman for the Automotive Oil Change Association, the 2018 National Oil and Lube News annual survey shows that more than half of car owners are choosing synthetics or synthetic blends when they get their oil changed.

Type of Oil	Percentage of Oil Changes Performed
Semi-synthetic (or blend) oil	35
Conventional oil	32
Full synthetic	22
High-mileage oil	6
Diesel	5

Should you use synthetic oil? There are good reasons to—but only if your car has specific needs.

Synthetics have some advantages over conventional motor oil. They’re designed to be more effective at:

Resisting oil breakdown, which makes it last longer than conventional oil
Withstanding higher temperatures than conventional oil, which helps keep engines running longer
Flowing in cold temperatures, thus reducing engine wear during frigid startups.

There's a downside: Synthetic motor oil can cost two to four times as much as regular oil. So unless your owner's manual specifies synthetic, you don’t need it.

But John Ibbotson, Consumer Reports’ chief mechanic, says there are some situations where synthetic oil’s resistance to breakdown (the tendency of oil to degrade and lose its viscosity over time) can help prolong the life of an engine:

If you make lots of short trips, standard motor oil may never get warm enough to burn off moisture and impurities that can accumulate. That could hasten the breakdown of conventional oil.
If you live in a region with very cold winters or very hot summers, or if you use your vehicle for towing or hauling heavy material, synthetic oil helps protect the engine from strain and won’t break down as quickly as conventional oil.
If you have an older engine that’s prone to sludge buildup. This gunky residue forms when oil breaks down, and it can block oil passages and lead to a quick engine death. In the early 2000s, several engines from Chrysler, Toyota, and Volkswagen, among others, were especially prone to sludge buildup. Synthetic oil is less likely to form this troublesome sludge.

Though synthetics generally hold up better for more miles, regular oil changes remain important, and you shouldn’t wait beyond the time interval recommended by the manufacturer—typically every six months or a year.

Using synthetic in these situations will prolong your oil life and require fewer changes. That’s also a major benefit to the environment, as used motor oil is a major source of toxic waste in water.

Patrick Olsen

I've been a journalist for decades, and I got my love of cars from growing up in Los Angeles, where you have to drive, even if you're just going a few doors down the block. I'm a huge movie, music, and TV fan, and really just a devotee of good storytelling.

Myth #1: The “W” in 10W-30 stands for “weight.”

The numbers in a multi-grade rating represent the oil’s viscosity, or thickness. The higher the number, the thicker the oil. The best oils are a happy medium; neither thick like sludge nor thin like water.

Because oil viscosity changes with temperature, the multi-grade rating gives you both the hot and cold viscosities. 10W represents the oil’s viscosity rating for winter use, according to a rating system developed by the Society of Auto Engineers. And that’s why the “W” in 10W-30 stands for “winter.” Find the right oil viscosity for your vehicle with the Cenex lubricants recommendation tool.

Myth #2: You should change your oil every 3,000 miles.

Oil chemistry and engine technology have evolved tremendously since the days of the 3,000-mile oil change. The majority of vehicle manufacturers now recommend vehicle owners change their oil every 7,500 to 10,000 miles, based on a normal maintenance schedule. Why the change?

Synthetic oils, such as Maxtron® PCMO, are helping to stretch the time between oil changes. The improved oils provide superior anti-wear protection while maximizing fuel economy.

Also, oil life monitoring systems, which notify the driver when an oil change is needed, are becoming the standard in many new models. Instead of using the standby miles driven method, the technology monitors engine revolutions, operating temperatures and other factors to optimize the change interval selection.

Refer to your owner’s manual for your vehicle manufacturer’s oil change recommendation.

Myth #3: Synthetic engine oils can wear down seals in an engine and cause leaks.

This is an often-cited myth. In fact, if your seals and gaskets are in good condition, synthetic oil will not leak in your engine. Synthetic oil has not been shown to deteriorate engine seals or gaskets. But it might find an existing leak. The smaller molecules of synthetic oil are able to pass through very small cracks and crevices that the larger molecules of petroleum-based oil cannot. Eventually, those small cracks and crevices can lead to bigger problems — with or without synthetic oil.

Myth #4: My engine will benefit if I use supplemental additives.

Adding commercially available additives to your oil is unnecessary, as reputable motor oils will be already formulated with additives to ensure optimal engine performance.

Additional additives may dilute the effectiveness of the oil or even upset the oil chemistry. Refer to your owner’s manual for information about oil additives for your vehicle.

Myth #5: Once you use synthetic oil, you can’t switch back to conventional oil.

Switching between synthetic and conventional oils will not cause any damage to an engine. In fact, synthetic blends are a mixture of synthetic and conventional oil.

Click here for more information on Cenex engine oils. Find the Cenex oil that’s right for your vehicle with our lubricants recommendation tool.

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What happens if you use regular oil instead of synthetic?

The differences mostly come down to the oil refining process. Conventional oil, while cheaper than synthetic, is less refined and can lead to more engine sludge and gunk build-up down the road.

Can I put conventional oil in a synthetic car?

Yes. There is no danger mixing synthetic and conventional motor oil. However, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.

Will the wrong oil damage my engine?

Cause damage to engine life The wrong engine oil can cause engine damage because it does not properly lubricate its components. This will lead to metal-on-metal contact since the moving parts are not being lubricated. Friction quickly follows, causing engine parts to wear.