What's the Difference Between Conventional and Synthetic Oil?

Engine oil changes are perhaps one of the first things that come to a person’s mind when they think of regular maintenance, and indeed it is the most-common maintenance item every single car needs, every 5,000 to 10,000 miles . Aside from oil viscosity, usually specified by the automaker, another common question is often raised: “Conventional or synthetic oil?” What is the difference between conventional and synthetic oil? Are there any benefits one way or the other? Is it worth the extra cost? To start with, the difference between conventional and synthetic oil is all about origins.

For this origin story, we need to go back several hundred million years, maybe a billion, before the dinosaurs roamed the earth – wait, before ? Yes, long before dinosaurs roamed the planet, it’s been theorized mats of bacterial colonies, millions of tons of microscopic life, lived and died in Precambrian seas. In fact, University of Georgia researchers estimate that the dry biomass of all bacteria on earth weighs in somewhere between 350 and 550 billion tons, about 3,000 times the mass of earth’s 7.6 billion human inhabitants. As these massive microscopic colonies lived, multiplied, and died, they would eventually become covered by layers of sediment, entombing them for millennia.

Hundreds of millions of years under intense heat and pressure, in an anaerobic (oxygen-free) environment, would eventually convert this ancient biomass into what we know of today as crude oil. Crude oil was discovered about 2,600 years ago, but it wouldn’t be until the 19 ^th Century that refining would reveal its multiple personalities, resulting in dozens of useful petroleum-based products, including gases, fuels, and lubricants, to name a few. Conventional motor oil is just one of these.

As the term suggests, synthetic oil is “synthesized” from other chemical bases. It might seem like a relatively new motor oil, but it was developed nearly 80 years ago, during WWII. In Germany, the chemical processes were used to synthesize lubricants and fuels, liquids, from rich coal deposits (solids). Modern synthetic oil is typically synthesized from CH ₄ (methane), CO (carbon monoxide), and CO ₂ (carbon dioxide), among others.

While actual “synthetic oil” is derived from chemical processes, and not from refined petroleum deposits, marketing synthetic oil isn’t so straightforward. When looking at API (American Petroleum Institute) base oil classifications, it’s good to note that Group I, II, and III motor oils are made from petroleum bases, which makes them conventional oils. Group III, IV, and V motor oils are marketed as synthetic oils, but only Groups IV and V are true synthetics. Still, it’s good to note that Group III oils are sufficiently processed, beyond simple refining, to render them chemically-similar to true synthetics.

When you buy motor oil, it’s quite different than when it was originally produced, in a refinery or chemical plant. Take, for example, a modern multi-viscosity 5W-30 conventional motor oil, which starts out as an SAE (Society of Automotive Engineers) Grade 5 oil. A soup of additives, specific to the application, modifies how the resulting motor oil blend reacts to heat. A 5W-30 conventional oil flows like an SAE Grade 5 in cold weather, easing engine starting and improving cold weather efficiency. The same blend, thanks to its additive load, thickens up as the engine heats up, flowing like an SAE Grade 30 oil at high temperature, maintaining oil pressure and lubricant qualities.

Unfortunately, because of its source and additive package, this 5W-30 conventional oil blend, up to 40% additives, has a limited lifespan as a 5W-30. Over time, through hundreds of heating and cooling cycles, the additives vaporize or oxidize, leaving the base SAE Grade 5, whose impurities also gradually oxidize and vaporize. The resulting “worn-out” oil can’t adjust to temperature changes or flow the way it did when it was new and whole.

Synthetic oil, on the other hand, is chemically structured to match a comparable multi-grade motor oil. Because it has fewer additives, there is less to oxidize and vaporize. Even after many more cycles in all kinds of running conditions, a 5W-30 full-synthetic oil doesn’t degrade as much as a comparable conventional oil blend. Over the useful life of a synthetic oil, its lubricant performance stays fairly consistent, from mile 1 through mile 10,000.

On asking the question, “Conventional oil or synthetic oil?” seeing any coupon or service offering, or walking through an autoparts store, you’ll immediately notice one major difference: price. In general, full-synthetic oil is 50% to 100% more expensive than a comparable conventional oil blend, but is it worth it? No matter what you’ve heard from friends and family, automaker and independent studies all show synthetic oil a clear winner in keeping engines running better and longer than conventional oil. Indeed, there’s a reason many automakers are assembling and shipping their vehicles with a fresh charge of synthetic oil.

As if that wasn’t enough, a recent AAA ( American Automobile Association ) study revealed exactly how much more synthetic oil changes cost, in comparison to how much benefit they get from them. First, what exactly does synthetic oil do for your engine, whether you’re driving a commuter, taxi, or sports car? Without getting technical, AAA’s independent study revealed that synthetic oil outperforms conventional oil by a margin of nearly 50%. This translates to less vaporization, less oxidation, less sludge, more consistent viscosity, and lasting lubricant quality. As a result, engines running synthetic oil perform better, run more efficiently, and last longer .

So, engines perform better and last longer with synthetic oil, but how much does it cost for such protection? Again, AAA’s study concluded that the price is well within most car-owner’s budgets. Setting aside monthly funds for regular maintenance and unexpected repairs, adding an extra $5 per month isn’t all that much to make the switch and take advantage of the benefits of synthetic oil at every oil change.

In the end, it’s a personal decision, but you should stick to automaker recommendations whenever possible. Some vehicles require synthetic oil, but any vehicle can benefit from it.