When it's time for an oil change, you're often presented with a big decision: conventional or synthetic motor oil. There is a lot of confusion surrounding synthetic oil, how it's made, and if a car really needs it. There are plenty of myths and half-truths surrounding this pricey lubricant. 

To help you decide which oil to go with, we will clear up the age-old question of what is synthetic oil and does my car need it?

What is Synthetic Motor Oil?

Synthetic oil actually starts its life as normal petroleum, so it's not completely synthesized. To make a full-synthetic motor oil base, the manufacturer breaks down the petroleum molecules and rebuilds them so they are all the exact same size and structure with fewer impurities. 

So, under a microscope, a synthetic oil base has millions of same-sized molecules, whereas a conventional oil or mineral oil base has millions of different-sized molecules. 

When a manufacturer creates full-synthetic motor oil, it starts with this base of perfectly sized molecules and installs the additives it needs to prevent the oil from breaking down. However, there is not a drop of actual petroleum in synthetic oil — only the molecules rebuilt into a synthetic base. 

Does My Car Need Synthetic Motor Oil?

No, your vehicle doesn't need synthetic oil in nearly every case. There are some exceptions in higher-end vehicles, like BMW or Mercedes, and performance applications where the manufacturer recommends synthetic oil. In this case, yes, you need it to maintain the manufacturer's warranty. 

Most cars, however, can run on either conventional engine oil or synthetic engine oil. 

Is There a Benefit to Using Full-Synthetic Engine Oil?

Yes, there absolutely is a benefit. In fact, there are a few key benefits when using synthetic motor oil.

Less Chemical Degradation

Synthetic motor oil is less prone to chemical degradation when exposed to the elements inside an internal combustion engine. These elements include:

  • Combustion byproducts
  • Fuel contamination
  • Water contamination
  • Metal shavings
  • Acids
  • Extreme heat

This means it's less prone to cause sludge buildup, varnish, deposits, corrosion, or lowered engine performance. It's also less prone to viscosity changes when put under these conditions. This means a cleaner and potentially healthier engine over the long term.

Higher Viscosity Index

Synthetics have a higher viscosity index when compared to conventional engine oil. This means as the temperature inside the engine changes, the oil's viscosity remains consistent. This means the oil remains thicker at high temperatures, leading to better lubrication, heat absorption, cleaning, and engine protection. 

Superior Oil Flow in Low Temperatures

When an engine is cold, the oil thickens, which is normal. In extreme cold, conventional oil can become so thin it won't circulate properly and potentially lead to premature engine wear. Synthetic oil is far less prone to this thickening and quickly circulates through the engine at startup, offering better protection of internal engine parts upon initial startup. 

Doesn't Burn as Easily

Regardless of what your brother's uncle says, all cars burn oil. This is why you sometimes run a little low in even a new car. Synthetic oil has far less volatility than conventional, leading to less burning. This keeps your oil level more consistent and is better for the environment. 

Longer Lifespan

Due to slower degradation, synthetic oil has a longer lifespan than regular oils. Many synthetic oils advertise 8,000-11,000 kilometre oil-change intervals. Some top-line brands even advertise up to 40,000 km oil-change intervals. 

There is one caveat: Oil-change intervals are determined by the vehicle, not the oil used. So, if your vehicle's manufacturer recommends 3,000-mile oil-change intervals in the owner's manual, you should continue following that regardless of the type of oil you use.

Are There Any Reasons Not to Use Synthetic Engine Oil?

There are a few small downsides to using synthetic engine oil, but none are overly alarming. 


Far and away the biggest issue with synthetic oil is its cost. Synthetic engine oil is often twice the price of mineral oil (or more). And since using synthetic oil doesn't change your oil-change interval, the immediate benefit of the higher cost isn't realized, leading most car owners to walk right by it. 

Long-Term Storage

The additives used in synthetic oils tend to separate from the oil in cold storage. You can rectify this with a good shake of the bottle, but it can be alarming to someone who's never seen separated oil. 

Reduced Fuel Efficiency

Mineral oil changes viscosity easier than synthetic oil, so it creates less resistance in the engine at higher temperatures. This reduced resistance in your car's engine means your vehicle may get slightly better fuel economy with conventional oil than synthetic. 

Synthetic Oil vs. Semi-Synthetic Oil

Go to any parts store, and you'll see a cheaper synthetic option sitting there: semi-synthetic or synthetic blend oil. This is when the oil company blends synthetic and conventional base oils when creating motor oil. 

There is no defined ratio of synthetic base to conventional base to go by, so it's difficult to say just how well semi-synthetic oil works. That said, a high-quality, well-blended semi-synthetic oil will provide superior protection relative to a conventional oil while costing far less than a full-synthetic oil. 

If synthetic oil is out of your budget, a quality semi-synthetic or synthetic blend oil is a great option.

Synthetic Motor Oil FAQs

Some common questions and misconceptions surround synthetic motor oil. Let's clear these up now. 

Can I Mix Synthetic and Conventional Oil?

Because synthetic motor oils are almost always polyalphaolefin (PAO) lubricants, they are exceptionally compatible with mineral oils. Mixing them in any ratio will have no impact on your engine, but it may reduce the effectiveness of the synthetic oil. 

Once I use Synthetic Oil Can I Switch Back to Conventional Oil? 

Yes, nothing prevents you from switching between these two oils at oil-change intervals. In fact, this can be a great way to enjoy the benefits of synthetic oil without always paying a significantly higher price. 

Will Synthetic Oil Make My Car Leak Oil?

No, synthetic oil will not cause an oil leak. However, if there is a spot where oil may leak, synthetic oil is thinner and more likely to flow from that spot than conventional oil. So, switching to synthetic may expose oil leaks, but it will never cause them. 

Is Synthetic Oil Worth the Extra Cost?

While you may not extend your oil-change intervals by switching to synthetic oil, there are other benefits. The biggest benefit is how clean synthetic oils are, making them less likely to cause buildup and varnish inside the engine. This buildup and varnish can shorten an engine's life. 

Plus, the hardest an engine has it is at initial startup, when there is little oil lubricating it. With synthetic oil's superior flow, your engine gets the lubrication it needs faster, reducing wear and potentially extending the engine's life. 

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