Synthetic Oil: What You Need to Know


Synthetic oil is a lubricant made up of artificially made chemical compounds; these compounds are made by breaking down and then rebuilding petroleum molecules.


What Is Synthetic Oil?

Synthetic oil is a lubricant made up of artificially made chemical compounds; these compounds are made by breaking down and then rebuilding petroleum molecules. Under a microscope, a drop of synthetic oil shows millions of molecules all nearly the same size and structure. Conversely, mineral or conventional oil is made using refined crude oil. A drop of conventional oil under a microscope shows millions of molecules all with different shapes, sizes and structures. Synthetic oil can be fully synthetic or a synthetic blend and be derived from multiple base types.

There are multiple types of synthetics with distinctly different properties and applications. However, most synthetics used in automotive service are polyalphaolefins (PAO). For simplicity, the primary reference to synthetic oils in this article will relate to PAOs.


Synthetic vs. Conventional Oil

Most vehicles are capable of using either synthetic or conventional mineral oil that meet the American Petroleum Institute's (API) and International Lubricant Standardization and Advisory Committee (ILSAC) specifications. However, synthetic oils are often marketed as having superior performance when compared with conventional oil. This superior performance may only relate to certain properties but not others. It is possible that some formulations of conventional mineral oils may exhibit superior performance on certain properties.

Among other things, AAA found that, on average, synthetic oils outperformed conventional oils by 47 percent in the conducted tests. The selected tests evaluated several important physical, chemical and performance properties including shear stability, deposit formation, volatility, cold-temperature pumpability, oxidation resistance, and oxidation-induced rheological (viscosity) changes.

Synthetic oil is quickly becoming the new normal, with nearly 70 percent of new cars in the 2019 model year getting either fully synthetic or a synthetic blend oil, according to Consumer Reports. Even as the number of new cars requiring synthetics continues to increase, it remains important for consumers to follow manufacturer recommendations for their vehicle when it comes to changing their oil. "Semi- or full-synthetic oils are required for most newer cars, but it is vital that the automaker's recommendations be followed, or accelerated engine wear and other problems could result," Michael Calkins, Technical Services Manager for AAA tells Machinery Lubrication.

The advancement of synthetic oil has put the microscope on conventional oil. Even though conventional oil lubricates your car's engine adequately in most cases, there are some possible disadvantages to using it. These disadvantages may or may not be realized or noticed depending on various factors like marginal fluidity at extremely low temperatures, thermal and oxidative stability (prolonged chemical stability at high temperatures) and viscosity protection (against wear and friction) at high engine loads and temperatures.


Advantages and Disadvantages of Synthetic Oil

So, what are the advantages of switching to a synthetic as opposed to a conventional motor oil? When looking at the pros and cons, synthetic lubricants can offer meaningful advantages. The following are some of the most significant advantages synthetics (PAOs) generally have over conventional engine oils. Note, the list below assumes that the additive package would be similar between the two options which is rarely the case. Most synthetic lubricant formulations for automobiles would have a superior additive package making the comparison more complex. In other words, is the superior performance the result of the synthetic base oil or is it due to the superior additive(s) or a little of both?

  •  When exposed to certain conditions, conventional mineral oils are usually more prone to chemical degradation (oxidation) compared to synthetics. These harmful conditions include combustion byproducts, fuel contamination, water contamination, metal particles, acids, pro-oxidants and extreme heat (e.g., from combustion). Exposure to these conditions commonly occur in engines. Oil degradation can cause sludge, varnish or deposits, corrosion, viscosity change and impaired engine performance.
  •  Synthetics have a naturally higher viscosity index. This means the viscosity changes less (more stable) as temperature changes during normal engine startup and operating conditions. Viscosity is an important property of lubricants that produces the film thickness or clearance between metal surfaces that slide or rotate against each other. Without this film thickness, excessive friction and wear would occur.
  •  At extremely low temperatures, it is more possible for mineral oils(compared to synthetics) to become so thick (high viscosity) that the oil is unpumpable or is unable to circulate effectively within the engine. Lack of oil circulation can cause lubricant starvation conditions and engine failure.
  •  Synthetic engine oils are generally less volatile than mineral oils. This means there is less loss of the oil to the engine's exhaust stream causing atmospheric pollution. This could also mean less need for makeup oil between oil changes.
  •  Possibly the biggest advantage and the reason synthetic oil is so popular, is that it has a longer lifespan than conventional oil. The recommended change interval for synthetic oil is around every 5,000 to 7,000 miles, with some brands touting a much longer interval (15,000 to 25,000). The biggest reason for the longer oil change interval is described in the first bullet above. Regardless of the kind of oil you use, it is still recommended to change your oil at regular intervals recommended by your manufacturer.

A few disadvantages of synthetic oil to be aware of include:

  •  Probably the most glaring downside of synthetic oil is the cost. The price of synthetic oil is around two to four times the price of conventional oil.
  •  Synthetics may be more prone to additives precipitation during cold storage conditions. This stratifies certain additives which can potentially lead to their complete separation from the oil.
  •  Multi-grade motor oils using synthetics may exhibit slightly less fuel economy at highway speeds compared to mineral oil. This is due to the fact that mineral oils require more viscosity index improvers (an additive) than synthetics. This additive contributes to reduced viscosity friction by a mechanism known as shear-induced temporary viscosity thinning.
Advantages and Disadvantages of Synthetic Oil
Advantages Disadvantages
Resistant to oxidation and chemical degradation More expensive
Withstands temperature extremes better (cold and hot) Possible additive precipitation/separation
Flows better at cold temperatures Slightly lower fuel economy at highway speed
May produce less sludge and surface deposits
Longer oil change intervals
More robust film thickness at higher temperature and higher loads

Is Synthetic Oil Worth the Cost?

When it comes to the price of synthetic or synthetic blends, it's not always black and white. "It is very difficult to generalize because of the many differences in oil brands and formulations. A quart of modern semi- or full-synthetic oil usually costs somewhere between four and 10 dollars, although some special blends can cost even more," Calkins explains.

As of 2019, five quarts of conventional oil costs approximately $28 and an oil change with conventional oil costs about $38, according to a survey of AAA's approved auto repair facilities. Conversely, five quarts of full synthetic oil will set you back about $45, while an oil change with full synthetic oil costs about $70. This means switching from conventional to synthetic will cost the average driver about $64 per year or $5.33 per month.

When it comes to the cost of an oil change, the change frequency of synthetic oil is longer, with most brands recommending an oil change at around 5,000 to 7,000 miles (up to 25,000 miles depending on some brands). Conventional oil changes are recommended at anywhere from 3,000 to 5,000 miles. It's important to note, if you're going to use a synthetic oil for longer oil change intervals, you should upgrade your oil filter to one also with a longer service life to match that of the synthetic oil.

So, is synthetic oil better for your engine than conventional mineral oil? "In general, yes – provided the automaker's viscosity grade and other oil requirements are adhered to," says Calkins. While mineral oil does provide adequate lubrication for your vehicle's engine, synthetic oil better protects your engine through the use of higher-quality, more refined base oils.

"Always use at least the type of oil recommended by your vehicle's manufacturer. If your car only calls for conventional oil, the slightly higher cost of a semi-synthetic product will provide better engine protection. Regardless of the oil used, always follow the vehicle manufacturers' recommended oil change intervals," says Calkins.

Additionally, think about how long you plan to own your car. Investing in premium lubrication may slightly extend the life of your engine; However, if you are planning to sell your car long before engine failure is likely with mineral oil, the benefit of using expensive synthetics is transferred to the next owner of your car. In other words, you paid for expensive oil but gained minimal benefit during the time you owned the car.

It should be said there are certain exceptions to this concept. One in particular is exposing an engine to worst case scenarios long before their normal end of life. Worst case scenarios include extreme temperatures, forgetting to change the oil on schedule, heavy loads, frequent cold starts, frequent short commutes, impaired engine cooling, etc.


Synthetic Engine Oils: Digging Deeper

Synthetic oil used in motor vehicles is refined from base oils or base stock oils. The American Petroleum Institute (API) categorizes base oils into five categories; the first three are oils refined from petroleum crude oil. Group IV base oils are fully synthetic, and Group V contains all base oils not in the first four groups. The API identifies two of the most common base oils used for synthetic oil as Polyalphaolefins (PAOs) and Esters (Diester and Polyol).

  •  PAOs are the most common type of synthetic base oils used today thanks to their moderate price and little negative attributes. PAOs are Group IV base oils and are similar to mineral oil in their chemical makeup but the fact that it's built rather than extracted makes it purer. The benefits of PAOs include improved oxidation and thermal stability, low volatility (potential to change rapidly), good heat dissipation, low pour point (temperature below which a liquid loses the ability to flow) and it's free of waxy molecules.
  •  Esters- a Group V base oil made from ester oils is created from the reaction of acids and alcohols with water molecules splitting off. The most common esters used for synthetic motor oil are diesters and polyol esters. Diesters are commonly used as an additive with PAO base stock oil. Esters have high thermal stability and excellent low pour point characteristics but poor hydrolytic stability (the resistance of a cured polymer material to going back to a semisolid or liquid form when exposed to high temperatures and humidity).
Synthetic Option Advantages Disadvantages
Poly Ester or Diester
(Group V)
Highest VI, Lowest Pour Point, Highest Thermal Stability May not be suitable where moisture accumulation is a problem. May not bear API service classification marks.
(Group IV)
Excellent VI, Excellent Pour Point, Excellent Thermal Stability May cause leakage in some instances. Additive stability in cold temperatures for extended periods of time may be a problem.
There have been numerous studies over the years comparing synthetic oil to conventional mineral oil. Most notably the American Automobile Association (AAA) used certified labs using American Society for Testing and Materials (ASTM) standardized test methods to examine differences in engine oils marketed as conventional versus those marketed as full synthetic.While a manufacturer-approved conventional oil won't harm your engine in any way, forking over the extra $30 per month and switching to a synthetic oil could save you money long-term in the way of protecting your engine's components over time. You may also make up for some of the extra costs by changing your oil less frequently.