Synthetic Versus Conventional Car Engine Oil

Spoiler alert! If you’re here to determine which is better for your car between synthetic car engine oil or conventional motor oil, here’s your answer: Synthetic oils are better than conventional oils.

That is not to say that conventional oils will not do an adequate job of lubricating your vehicle’s engine. As long as an engine oil fulfills applicable lubrication standards like that of the American Petroleum Institute (API) oil service categories, you can count on it to do the job it’s supposed to do.

However, some engine oils do a better job or have an advantage over others.

For instance, some perform better for longer — leading to longer oil change intervals. Case in point, synthetic oils typically last longer than conventional oils. If you want to know why that is, feel free to read on.

What Are Conventional Oils?

Car engine oils are a combination of base oil and performance enhancement additives. The base oil is the main ingredient of any lubricating oil as the base oil comprises from 80% to 99% of most engine oil formulations.

The American Petroleum Institute classifies all base oils into five groups. Conventional lubricants are car engine oils made from base oils manufactured by refining petroleum crude oil. They are Groups I, II, and III base oils.

Since conventional oils are refined crude oil, they are also known as mineral oils.

Caveat: In some countries, engine oils with Group III base oils may be marketed as synthetic oils.

What Are Synthetic Oils?

If conventional or mineral engine oils are made with Groups I, II, and III base oils, synthetic oils, as a general rule, belong to Groups IV and V. In practice, engine oils dubbed or labeled ‘fully synthetic’ are typically made using Group IV base oils.

Group IV base oils are synthesized in the laboratory, unlike Group I to III base oils. Group V base oils are typically synthetic, but there can be mineral Group V oils as well.

Groups of Base Oils

At this point, it makes sense to discuss the API’s base oil classifications. What makes a Group I, Group II, Group III, Group IV, and Group V base oil?

Group I

Group I base oils are neutral oils with a saturate level of less than 90%, a sulfur level greater than 0.03%, and a viscosity index of 80 to 120. Group I base oils are manufactured through solvent refinement.

Solvent refining is a liquid extraction process. The most commonly used liquid extraction agents are furfural, phenol, and sulfur dioxide.

Group II

Group II base oils are hydro-processed crude oils. They have greater than 90% saturates, less than 0.03% sulfur level, and a viscosity index of 80 to 120.

Hydrocracking is a process used to manufacture Group II base oils.

In hydrocracking, hydrogen is added to the base oil as it flows over a bed of catalyst at temperatures greater than 343 degrees Celsius and at pressures greater than 1,000 psi. The resulting chemical reaction leads to the reshaping of some hydrocarbon molecules and the cracking of larger molecules into smaller molecules.

Group III

Group III base oils have greater than 90% saturates, less than 0.03% sulfur level, and a viscosity index greater than 120. Essentially, they are Group II oils with a higher viscosity index.

Base oil manufacturers use severe hydrocracking to manufacture Group III base oils. Severe hydrocracking uses a higher temperature and higher pressure, leading to more severe reshaping and cracking of hydrocarbon molecules.

Because of the high viscosity index of Group III oils, they are highly stable and high-performing oils. As such, although car engine oils made with Group III base oils are technically mineral oils — since a Group III oil is refined petroleum crude oil — engine oils made of Group III base oils are sometimes marketed as synthetic oils.

Group IV

Group IV base oils are not mineral oils because they are not refined from petroleum crude oils. Instead, they are synthesized in laboratories.

Group IV oils are 100% polyalphaolefins or PAOs, which are designed from organic alpha-olefins. Since they are synthesized, they have a uniform molecule size and high viscosity index.

Group V

Group V is a catch-all category for base oils that do not fit into the other groups.

Typically, Group V oils are synthetics but non-PAOs; they can be made of esters, polyalkylene glycol, and silicone. However, Group V may include mineral oils, such as the pure white oil used in food processing and cosmetics.

Group V oils are typically used as base oil additives rather than as the main ingredient in engine oils.

Why Are Synthetic Base Oils Better Than Mineral Base Oils?

Generally speaking, the higher an oil is in the API group classification, the more refined it is.

The more rigorous refinement processes the feedstock goes through as they go up the classification levels ensure a higher concentration of saturates, lower levels of sulfur content, and a higher viscosity index.

Saturates Level

The proportion of saturated hydrocarbon molecules in a base oil is a gauge of that oil’s molecular bond. More saturates mean a stronger molecular bond. This translates to better oxidative stability and resistance to breakdown.

Therefore, you want a base oil with a higher saturate level.

Sulfur Level

The presence of sulfurs, which naturally occur in crude oil, has advantages and disadvantages. It acts as an antioxidant, which helps base oils resist oxidation and thus degradation. It is also often used as a component of additives meant to improve oil’s anti-wear and lubricity properties.

However, sulfur’s reaction with oxygen (thus, its antioxidant function) leads to the formation of acids, which can lead to non-ferrous corrosion. Therefore, too much sulfur in base oils can be harmful to engines. So, you want a base oil with lower sulfur content.

Viscosity Index

The viscosity index is an indicator of the stability of the viscosity of base oils. All base oils become increasingly thick or more viscous in lower temperatures and increasingly thin or less viscous in higher temperatures.

In high viscosity index oils, changes in temperature lead to more minor changes in viscosity. In contrast, low viscosity index oils display more drastic viscosity changes with temperature changes.

Thus, the higher the base oil viscosity index, the better.

Synthetic Engine Oils Are Better

Indeed, synthetic engine oils, which are made from Group IV base oils, are better engine oils than engine oils made from Group I, II, and III base oils.

Group IV oils have high levels of saturates, low sulfur content, and high viscosity index. Consider that when it’s time for your next engine oil change.