Synthetic vs. Conventional Oil: The Definitive Guide

Find out everything you need to know about synthetic vs. conventional motor oil.

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by John Baker
June 8, 2020

In the battle of synthetic vs. conventional oil, most motorists know that synthetic oil emerges victorious.

It’s widely understood that synthetic oil provides improved wear protection, engine cleanliness and fuel efficiency, among other benefits. Synthetic oil also lasts longer, offering increased convenience and cost savings.

But, what makes synthetic oil better than conventional oil? Let’s find out.

Here’s what we’ll cover:

What is Synthetic Oil?

It’s a question motorists often pose. They also ask, “What is synthetic oil made from?”

Some erroneously think synthetic oil is not derived from crude-oil sources or other fossil fuels. They think that if all motorists switched to synthetics, we’d sever our ties to fossil fuels and simultaneously save the children, polar bears and whales.

A noble idea, but a false one.

While synthetic and conventional oils differ in performance and how they’re made, they both owe their origins to crude oil or other fossil fuels.

But, as you’ll see, they are still vastly different.

Motor oil, whether conventional or synthetic, is made from two fundamental components:

The base oils are primarily responsible for fighting wear, reducing heat and minimizing friction. The addition of different chemical additives to the formulation enhances motor oil performance. Additives combat chemical breakdown, neutralize acids, provide additional wear protection and more, depending on the formulation and application for which it’s intended.

To illustrate, think of a quart of oil like a glass of lemonade. The base oils are the water and the additives are the lemon concentrate and sugar.

To answer our initial question (What is synthetic oil?), let’s forget the additives for a moment and focus on the base oils.

How Conventional Base Oils are Manufactured

The base oils used to make conventional motor oil are distilled from crude oil.

Crude oil contains hundreds of different hydrocarbons. Oil refineries distill the crude using heat, pressure and other catalysts to separate it into different products, including…

  • Gasoline
  • Jet fuel
  • Diesel fuel
  • Heating oil
  • The base oils from which conventional motor oil is made

Distillation, however, has its limits. Base oils distilled from crude contain several different molecules that are detrimental to lubricating a vehicle’s engine. As a result, oil performance suffers – and so does your engine. In fact, the limits of distillation became evident during a famous WWII battle, which helped defeat the axis powers while speeding development of synthetic oil. Read about the history of synthetic oil here.

How Synthetic Base Oils are Made Differently

Synthetic base oils aren’t distilled; instead they’re chemically synthesized (i.e. built).

What does that mean?

Refiners disassemble crude oil molecules to their fundamental components using various chemical reactions. Then, using only uniform molecules (typically ethylene), they build synthetic base oils from the ground up. What results is a pure base oil that doesn’t contain the mishmash of irregular molecules found in conventional base oils distilled from crude oil.

This is all a bit confusing unless you’re a chemist, so let’s use another analogy.

Say you bought an old Victorian house. It’s loaded with character, but it’s seen better days. The foundation is cracked, the plaster is worn, the walls are out of plumb and the roof leaks. You decide to make repairs.

There are two ways to go about it: the conventional way and the synthetic way.

The Conventional Way

You fix the foundation, patch the cracked plaster, repair the worn shingles and decide to live with the crooked walls. It’s not a perfect renovation, but it’s economical and the house should provide reliable shelter for a few years before needing more repairs.

The Synthetic Way

Rather than papering over the glaring problems, you disassemble the house piece by piece.

You remove each shingle, piece of siding, nail, brick, stud and joist. Soon, all the house’s components are spread across the yard.

Next, you rebuild the house from the ground up using only the straight studs and joists, good bricks, pristine shingles, etc. The final product is far stronger and more robust than the house renovated the “conventional” way.

In the same manner, formulators tear down crude oil molecules into fundamental components and, via organic synthesis, build synthetic base oils using only the molecules that are “straight, square and in excellent condition,” so to speak. The resulting product is optimized for protecting your engine against wear, heat and stress.

This analogy applies to polyalphaolefin [PAO]-based synthetic base oils, and not synthetics made from Group III base oils. Not all synthetic oil groups are created equal.

Is Synthetic Oil Better Than Conventional Oil?

Yes. Thank you, next?

Seriously, though, if you’ve read this far you can arrive at the answer yourself.

Synthetic motor oil is made using pure, uniform chemicals. As such, it imparts benefits conventional motor oils just can’t touch. It makes perfect sense. Just about anything manufactured with better components is going to provide better performance and last longer – an automotive engine, smartphone, pair of shoes – you name it.

Synthetic Oil vs Conventional Oil Test

But, don’t take our word for it.

The American Automobile Association (AAA) conducted a study to determine if, in fact, synthetic oil outperformed conventional oil and was worth the extra up-front price.

Here’s what John Nielsen, AAA’s managing director of Automotive Engineering and Repair, said following the synthetic oil vs. conventional oil test:

“Oil protects critical engine components from damage and AAA found that synthetic engine oils performed an average of 47 percent better than conventional oils in a variety of industry-standard tests. With its superior resistance to deterioration, AAA’s findings indicate that synthetic oil is particularly beneficial to newer vehicles with turbo-charged engines and for vehicles that frequently drive in stop-and-go traffic, tow heavy loads or operate in extreme hot or cold conditions.”

Those conditions apply to just about any vehicle on the road today.

Our own testing versus conventional oil confirms the AAA’s findings. For example…

  • AMSOIL Signature Series 5W-30 showed 38% less oil consumption than the tested conventional oil, requiring less-frequent top-offs, in the ASTM D5800 Noack volatility test.
  • AMSOIL keeps engines 81% cleaner (5 times cleaner) than conventional oil by better resisting the formation of deposits (supported by TEOST 33C test [ASTM D6335]).

Benefits of Synthetic Oil

We’ve established (and so has the AAA) that synthetic oil outperforms conventional oil in testing. So, what benefits might drivers expect after switching to synthetic oil?

Improved Engine Protection

Wear protection is the number-one benefit motorists demand from motor oil. If motor oil could make your car fly, but couldn’t keep components from wearing out and failing, what good would it be?

Synthetic motor oil does a better job protecting against engine wear. It forms a stronger fluid film to help keep metal components from contacting and wearing against each other. It also offers improved resistance to heat and viscosity loss, two features that translate into better wear protection.

Want proof? Consider the performance of AMSOIL Signature Series Synthetic Motor Oil against a leading synthetic-blend motor oil (more on synthetic-blends later).

What About Protection in Diesel Engines?

Again, the evidence shows the superiority of synthetic motor oil in guarding against wear in diesel engines.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil provides 6X more engine protection than required by the Detroit Diesel DD13 Scuffing Test for Specification DFS 93K222 using 5W-30 as worst-case representation.

Improved Cold-Temperature Performance

When the temperature drops, motor oil thickens and moves slower, causing parts of your engine to remain unprotected for a short period of time. It’s especially problematic with conventional oils since they contain waxes that thicken in the cold.

Synthetics, on the other hand, don’t contain waxes, meaning they remain more fluid in the cold. This means synthetics can reach vital components faster, providing more immediate engine protection and reduced wear. Simply said, synthetic oils offer better cold-weather protection.

See for yourself in the video.

Improved High-Temperature Protection

Engines get hot. In fact, they run hotter than ever thanks to the proliferation of turbochargers and direct fuel injection, which help automakers achieve today’s strict fuel-economy standards.

Turbos can spin up to 300,000 rpm, while the exhaust gases that drive the turbine can hit 1,000ºF (538ºC). The extreme speed combined with the blistering heat can lead to deposit buildup, known as turbo coking, which can ruin the turbo. Turbo coking occurs when oil is left in the turbo and the engine is shut off hot. The heat cooks the oil inside the turbo and leaves behind deposits.

Look at the image. That glowing chunk of metal is a turbocharger.

It’s up to the motor oil to prevent that from happening. And conventional oils simply aren’t cut out for the task. They contain unstable, light molecules that vaporize when exposed to extreme heat.

Synthetics, in contrast, offer improved extreme-heat resistance, thus they’re the better choice for today’s hot-running engines. In fact, turbo protection is so important that General Motors requires oils to pass its Turbo Coking Test to be recommended for its GM dexos1® Gen 2 motor oil specification.

We decided to put AMSOIL Signature Series Synthetic Motor Oil to the test. How did it perform? See for yourself.

Maximum Fuel Economy

Earlier we talked about how conventional oil thickens when it’s cold. Not only does thick, cold oil affect wear protection, it wastes energy. This, in turn, requires the engine to use more fuel. Because synthetics flow readily at start up, synthetic oils maximize fuel economy.

In addition, modern low-viscosity oils, like 0W-20, flow more readily than the 10W-40 oils vehicles once used. This, in turn, boosts fuel economy. But formulators must use synthetic base oils to manufacture low-viscosity oils that provide good protection in all areas important to your engine.

Reduced Oil Consumption

Several factors can cause your engine to increase motor oil consumption. Most are mechanical, including…

  • Leaking seals or gaskets
  • Stuck or worn piston rings
  • Worn or damaged bearings
  • Clogged PCV valve

If your engine uses oil due to mechanical defects, visit a mechanic and have them fixed.

However, motor oil volatility can also contribute to oil consumption. Volatility defines a lubricant’s evaporative loss. The more volatile the lubricant, the higher the tendency to evaporate and exit the engine through the crankcase ventilation system.

The more it evaporates, the less oil is left to protect equipment and the more often a user must replace the lost oil.

The small, light molecules in conventional lubricants evaporate at relatively low temperatures. These light molecules require less energy in the form of heat than heavier molecules to be lifted out of the solution and into the air.

Volatility Affects More Than Oil Consumption

When light elements in oil evaporate from heat, the oil’s viscosity increases. This thicker oil forces the engine to work harder, resulting in several problems, including the following:

  • Performance loss
  • Fuel economy loss
  • Poor cold-temperature starting
  • Increased engine deposits

The solution is to use an oil that’s stable in the presence of heat and resists oil consumption. Synthetic oils offer naturally better resistance to heat and volatility. As a result, they help reduce oil consumption to keep your engine clean and running strong.

For example, AMSOIL Max-Duty Synthetic Diesel Oil provides up to 76% less oil consumption than required by the API CK-4 standard in the Caterpillar-1N oil consumption test.

AMSOIL reduces oil consumption in gasoline engines, too, as this graph shows.

How Often Should I Change Synthetic Oil?

It stands to reason that synthetic motor oil will last longer than conventional oil due to its superior performance. That’s true, but how long does synthetic motor oil last?

It depends on the oil itself and your operating conditions.

While synthetics in general outperform conventional oil, some synthetics are better than others. That means some will last longer than others.

Most synthetics can last 8,000-10,000 miles. But some synthetics today are recommended for drain intervals up to 25,000 miles or one year. AMSOIL Signature Series Synthetic Motor Oil, for example, is guaranteed for 25,000 miles/one year/700 hours, whichever comes first, in normal service. Changing oil once a year reduces waste oil and packaging considerably. Plus, it reduces time and money spent changing oil.

Oil Quality Helps Determine its Service Life

How long synthetic oil can last depends on its base oil and additive quality.

It also depends on whether your driving conditions fall under the severe or normal designation.

Excessive idling; frequent towing, hauling or plowing; or driving in dusty conditions are examples of severe conditions that can reduce the oil’s drain interval.

We recommend that you follow the oil-change guidelines given in your owner’s manual or on the motor oil label.

Some synthetics are recommended for extended drain intervals beyond what’s given in the owner’s manual.

Extending drain intervals in this way allows you to safely change oil less often while spending less on oil over the long haul, if you choose.

AMSOIL Signature Series (mentioned above) is one example. Its diesel counterpart, AMSOIL Signature Series Max-Duty Synthetic Diesel Oil, is another. In certain applications, it’s recommended for up to 3X the original equipment manufacturer drain interval, not to exceed 60,000 miles/600 hours of service or one year, whichever comes first. Check out AMSOIL oil change intervals here.

Used oil analysis is the best method of determining how long your motor oil can last. It involves submitting an oil sample to a laboratory, where technicians analyze its condition for the presence of wear metals, coolant, fuel and other contaminants.

Oil analysis is the most efficient way to determine exactly how long your oil will last and when it needs changing.

Synthetic vs Conventional Oil Change Interval

Remember the 3,000-mile oil change? Some quick lubes and shade-tree mechanics may still promote the practice as “cheap insurance” against engine wear.

Wanting the best for your engine is a noble idea, but throwing out perfectly good motor oil benefits no one (except the company selling you the new oil).

The 3,000-mile oil-change interval was adopted and amplified in the 1970s by quick lubes (many of which were owned by oil companies). While they may have had their customers’ best interests in mind, it didn’t hurt that the practice also ensured repeat business.

Those days are over.

Just like vehicles no longer require carburetor adjustments, new ignition points or water in their battery, they don’t require oil changes every 3,000 miles (barring extreme operating conditions).

Motor oil technology has advanced in lockstep with automotive technology, meaning oils today commonly last longer than the traditional 3,000 miles. As mentioned above, some synthetics offer the ability to extend oil drain intervals if you choose, saving time and money.

Can You Mix Conventional Oil With Synthetic Oil?

Yes, you can mix conventional oil with synthetic oil.

There is no danger in mixing the two. Synthetic-blend motor oil, for example, which is readily available at any parts store, is nothing but conventional and synthetic oil already mixed in the bottle.

Mixing the two, however, will only detract from the synthetic oil’s superior performance and reduce its benefits.

It’s not a great idea to mix conventional and synthetic motor oil unless you have no choice. If you’re travelling, for example, and notice the engine low on oil, the convenience store alongside the road may only sell conventional oil. In these cases, we recommend draining the oil at your earliest convenience and refilling with synthetic motor oil.

What is Synthetic-Blend Motor Oil?

We alluded to the answer above: it’s nothing more than motor oil that’s formulated using both conventional and synthetic base oils. Synthetic-blend motor oil is also referred to as semi-synthetic.

Synthetic-blend motor oils fall in between conventional oil and synthetic oil on the motor oil spectrum. The idea behind them is to offer motorists a taste of the improved performance of a full synthetic at a price closer to a conventional oil. Oil marketers see them as a stepping-stone product to help encourage motorists to eventually step up to a full synthetic.

The biggest question surrounding synthetic-blend motor oil is, “How much ‘synthetic’ is in my oil?” Does the oil in the bottle include 1% synthetic base oils? 10%? 30%? There’s no way to know.

Check out this post for more on “How much ‘synthetic’ is in my oil?

The higher the synthetic content, the better the oil should perform.

Full Synthetic Oil vs Synthetic Blend

Unfortunately for motorists, there are no industry specifications or regulations that define the minimum percentage of synthetic components for a synthetic-blend to be classified as such. It varies depending on each manufacturer, and they’re not going to divulge that information since it would give their competition an advantage.

That being the case, astute motorists can look for a few clues to help interpret the amount and quality of the synthetic base oils in your synthetic-blend motor oil.

Visit the oil manufacturer’s website and find the oil’s technical data sheet (also called a “product data sheet”). Find the section labeled “Typical Technical Properties,” or some derivative of such. Look for these two values:

  • Viscosity Index: Oils with a higher number include a better synthetic base oil that provides more protection to critical components over a wide temperature range by maintaining fluid thickness and the necessary fluid barrier between parts.
  • Pour Point: measures the oil’s fluidity at cold temperatures and refers to the lowest temperature at which oil maintains its ability to flow. Lower numbers are likely to indicate a better synthetic base oil.

You should know, however, that an oil’s base oil composition is only part of what determines its performance. The additives also play a large role in fighting wear and deposits.

AMSOIL Performance Testing

Laboratory tested and proven on the road.

See how AMSOIL products perform in the lab and in the field.

Check out the tests

Oil Types Explained: Synthetic, Blend, Conventional & High-Mileage

We just explained what synthetic-blend oil is above (a mixture of conventional and synthetic motor oil).

And, you now know the differences between synthetic and conventional motor oil. (If you missed it, scroll back to the top of this post.)

But that leaves one more oil type to talk about: high-mileage oil. It’s the fastest-growing oil type available.

As engines age, seals can dry out and parts can wear if motorists haven’t be using a premium synthetic oil.

High-mileage oils contain extra additives designed to condition aging seals to help prevent leaks. Their viscosities also fall on the upper end of the spectrum to help seal the widening clearances between metal parts that are wearing over time.

Though most are synthetic-blend oils, full-synthetic oils designed for high-mileage engines (usually 75,000 miles [120,000 km]) are increasingly entering the market.

Often, the buyer’s motivation for using a high-mileage oil is similar to motorists who buy synthetic-blend oil – a desire for better protection, but at a lower price than top-shelf synthetics.

Synthetic Oil vs Conventional Oil in Older Cars

Misinformation about synthetic oil vs conventional oil in older cars still populates the Internet and some auto repair shops.

Some people still erroneously think that synthetic oil can be “too slippery” and result in leaking seals. Or synthetics will cause roller followers to slip or skid.

Like many popular misconceptions, the idea that synthetics cause oil leaks is based on truth.

Back in the 1970s, some synthetic oils were formulated using chemicals that were harder on seals than the chemicals in modern synthetics. Early formulations sometimes resulted in oil leaks, which gave rise to the myth that exists today.

Modern synthetics, however, are much more advanced and use chemicals that are fully compatible with seals. A properly formulated synthetic conditions seals and helps maximize their life and effectiveness.

How to Switch to Synthetic Motor Oil

Given its superior performance, many motorists are making the switch to synthetic motor oil for the first time. Several automakers install synthetic lubricants at the factory due to their improved protection, which further encourages motorists to use synthetic motor oil.

If you decide to opt for a full synthetic oil change next time your vehicle is due, here’s what you need to do first: nothing.

That’s right. You need not do anything special to switch to synthetic motor oil. Simply drain the conventional motor oil and install synthetic motor oil of the correct viscosity for your engine.

Consider an Engine Flush

Depending on the condition of your engine, you may want to perform an engine flush to remove sludge prior to switching to synthetic motor oil.

Synthetic Motor Oil Beats Conventional Motor Oil

Synthetic motor oil outperforms conventional oil on nearly all counts. It delivers excellent protection against wear, damaging deposits and chemical breakdown. Switch to synthetic motor oil to help your engine last longer and deliver maximum power and performance.

by John Baker

AMSOIL Technical Writer and avid avid DIYer with 12 years in the synthetic lubricants industry, who enjoys making technical topics in the automotive, powersports and industrial markets easy to understand.

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