Base Oil Categories

The American Petroleum Institute (API) developed a classification system for base oils that focuses on the paraffin and sulfur content and degree of saturation of the oil. The saturate level indicates the level of molecules completely saturated with hydrogen bonds, leaving them inherently nonreactive. There are five groups in the classification system, ranging from Group I to Group V. Figure 1 details the five groups by their manufacturing process, saturate and sulfur level and their viscosity index (VI). General group characteristics are listed below.

Group I Characteristics
Group I base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little uniformity. While some automotive oils use these stocks, they are generally used in less demanding applications.

Group II Characteristics
Group II base oils are common in mineral-based (non-synthetic) motor oils. They have fair to good performance in the areas of volatility, oxidation stability, wear prevention and flash/fire points. They have only fair performance in areas such as pour point and cold-crank viscosity.

Group III Characteristics
Group III base oils feature reconstructed molecules that offer improved performance in a wide range of areas, as well as good molecular uniformity and stability. By definition, they are synthesized material and can be used in the production of synthetic and semi-synthetic lubricants.

Group IV Characteristics
Group IV base oils are made from polyalphaolefins (PAO), which are chemically engineered synthesized base stocks. PAOs offer excellent stability, molecular uniformity and improved performance.

Group V Characteristics
Group V base oils are also chemically engineered stocks that do not fall into any of the categories previously mentioned. Typical examples of Group V stocks are esters, polyglycols and silicone. As with Group IV stocks, Group V stocks tend to offer performance advantages over Groups I to III. An example of a mineral-based Group V exception would be a white oil.

Defining Mineral Oil Properties

Mineral Oils are generally classified as paraffinic and naphthenic. The difference between paraffinic stocks and naphthenic stocks is one of molecular composition, resulting in inherent solvent differences between the two types of stock.

Paraffinic Stock
Paraffinic oils are characterized by straight chains of hydrocarbons where the hydrogen and carbon atoms are connected in a long linear composition, similar to a chain.

The wax matter within the paraffinic stock results in these elements tuning to solids at low temperatures; therefore, untreated paraffinic stocks do not have good cold-temperature performance and consequently, the pour point of paraffinic stocks is higher. In order for a paraffinic stock to flow at low temperatures, the heaviest waxes must be removed and usually pour-point depressants are necessary.

Paraffinic stocks display good high-temperature performance with high oxidation stability and high flash/fire points. Paraffinic stocks also have a high viscosity index (VI), meaning that they exhibit high viscosity stability over a range of temperatures.

Naphthenic Stock
Naphthenic Oil stocks are much like paraffinic stocks in that they contain only hydrocarbons. However, naphthenic stocks differ, and they are characterized by a high amount of ring hydrocarbons, where the hydrogen and carbon atoms are linked in a circular pattern. Conventionally, when the paraffinic carbon content of oil is less than 55-60 percent, the oil is labeled as naphthenic.

Naphthenic crudes contain very little to no wax and therefore will remain liquid at low temperatures; however, they will thin considerably when heated. Naphthenic stocks generally have low VI. These stocks have higher densities than paraffinic stocks, and they have greater solvency abilities than their paraffinic counterparts. Because naphthenic stocks contain little wax, they display lower pour points than paraffinic stocks. These stocks are also volatile and have a lower flash point.

Because naphthenic crudes contain degradation products that are soluble in oils, they present fewer problems with the formation of sludge and deposits. Due to the performance characteristics of naphthenic oils, they are generally used in applications where low pour points are required and the application temperature range is narrow.

Defining Synthetics

A true definition for the term synthetic oil has been difficult to reach, although it has generally been accepted that the term represents those lubricants that have been specifically manufactured for a high level of performance. In 1999, the National Advertising Division (NAD) ruled that Group III base oils with very high viscosity indices could be called synthetic oils.

The construction of a synthetic base stock will vary depending on the particular stock. While mineral stocks are derived through a distillation process, synthesized stocks are derived from a chemical reaction process. Synthetic lubricants are engineered for a specific molecular composition; they undergo a specific reaction process to create a base fluid with a tailored and uniform molecular structure. This allows chemists to develop lubricants with specific and predictable properties.

While and average mineral oil stock may possess a moderate amount of semi-beneficial molecular compounds, synthetic stocks, by design, can be composed completely of beneficial molecular compounds. Because of this, synthetic stocks are able to extend the service life of both oil and equipment, and they also have a wider range of acceptable temperature margins than conventional (mineral) stocks.

Oftentimes people misunderstand the term “synthetic lubricant”, believing it refers to one type of stock, when it in fact represents a number of oil stocks. While it can be generalized that all synthetic lubricants have superior performance capabilities over mineral oils, the variations in characteristics can be significant. One synthetic stock can be excellent for producing motor oils and drivetrain fluids, while others will be totally unacceptable for such applications.

The most common synthetic base stocks used in the transportation industry are PAOs, esters, and Group III mineral oils. Keep in mind that within each family name, additional groups may exist. For example, esters can be further divided into sub-categories of esters with varying properties.

Synthetic Hydrocarbons
Synthetic hydrocarbons are the fastest-growing synthetic lubricant base stock. Synthetic hydrocarbons are fluids that are formulated to specifically meet critical requirements and provide superior performance. These fluids often are made from a single type of molecule, usually of restricted molecular range. Such tailored fluids provide increased performance characteristics over petroleum (mineral) stocks.

Synthetic hydrocarbon base stocks can be used in combination to provide characteristics such as solvency, temperature performance, surface strength and volatility qualities.

Polyalphaolefins (PAOs)
Of all the synthetic base materials, PAOs are likely the closest relative to mineral oil stocks. Both types of oil stocks are comprised of similar hydrocarbon molecules; however, PAO stocks consist of a single molecular structure, whereas mineral oil contains a broad range of structures.

PAOs are commonly manufactured by reacting ethylene gas with a metallic catalyst. The major advantage of PAOs is their ability to function over a broader temperature range than their mineral-based counterparts. PAOs also provide good stability, which helps to reduce engine deposits. Correctly formulated PAOs have the ability to hold large quantities of contaminants in suspension, further reducing deposits.

Group III Oils
Group III oils undergo the most stringent level of refining for petroleum oils; most of the waxes and impurities naturally occurring in the oil are removed. The high level of refining gives Group III oils a high level of performance – in some instances outperforming PAOs. Since the ruling of the National Advertising Division (NAD) in 1999, Group III oils can be legally called synthetic oils. The decision was based on the amount of refining the oil is subjected to.

Esters
Esters are synthesized base stocks that date back to World War II. Esters were used to harness low-temperature performance to enhance mineral-oil blends. Esters are the product of combining organic acids with alcohols. Two common classes of organic esters are dibasic acid esters (diesters) and polyol esters. Another common class is phosphate esters; which have limited use due to their toxicity levels.

Dibasic Acid Esters (Diesters)
Dibasic acid esters are part of the ester family of synthetic base stocks. More commonly referred to as diesters, they are typically manufactured by reacting grain alcohol with a fatty acid catalyst. Their key advantages include the ability to function over broad temperature ranges, thermal and oxidative stability and exceptional inherent lubricity.

Polyol Esters
Polyol esters are also members of the ester family of synthetic base stocks. Commonly manufactured by reacting a fatty acid with polyhydric acids, polyol esters share the same broad operating temperature range as other synthetic base stocks and exhibit good thermal and oxidative stability.

Phosphate Esters
Phosphate esters are commonly manufactured by synthesizing phosphorus oxychloride and alcohol or phenols. While they offer fire resistance, their poor low-temperature performance and high toxicity limit their use.

Silicone Fluids
Silicone fluids are another type of synthetic stock used in specialty greases where performance over a wide temperature range is needed.

Polyglycols (PAGs)
Polyglycols, also referred to as polyalkylene glycols or PAGs, are a family of synthetic lubricants with varying product applications and properties. A major benefit of these fluids is their ability to completely decompose under high-temperature conditions, producing very little sludge. They have a tendency to increase in viscosity at low temperatures, but overall, they represent good viscosity-temperature properties.

Defining Additives

Oil additives are chemical compounds added to base stocks for the purpose of providing specific performance properties to the finished product. Specific properties are chosen based on the operating conditions and equipment type the oil will be used in. Today’s additive systems can be quite sophisticated, yet they can be chemically sensitive and negatively affected by the addition of other chemicals. Therefore, AMSOIL motor oils should never be intentionally mixed with aftermarket lubricant additives.

The role of additives is to perform two functions: enhance the oil’s beneficial properties and lessen the destructive processes in the oil.

Common additives include: pour point depressants, viscosity index improves, defoamants, oxidation inhibitors, rust and corrosion inhibitors, detergents and dispersants and anti-wear and extreme-pressure additives.