Is It a Synthetic?
It was easy to answer this question 10–15 years ago. At that time synthetic hydrocarbons (PAO — polyalphaolefin) were the bulk of the synthetic base stocks used in lubricant formulations for automotive and industrial applications. Often these synthetic lubricants included synthetic esters (the reaction product formed between organic acids and alcohols) to improve seal compatibility.
Ten to 15 years ago, the term partial-synthetic lubricant was used to describe mixtures of synthetic base stock (PAO — up to 30%) with conventionally refined petroleum oils produced by the solvent extraction refinery process.
In the 1980s, highly refined, very pure and colorless, hydrocracked petroleum oils entered the U.S. market. They offered significantly higher levels of performance than conventionally refined oils in automotive and industrial lubricant formulations. Early on, these base stocks were described as semi-synthetic base stocks.
After a federal court ruled that hydrocracked oils are also “synthetic”, a lot of “synthetic lubricant” formulations were introduced on the market formulated with high-performance petroleum oil base stocks. Costs for these base stocks are less than traditional “synthetics” but more than conventional refined petroleum oils. Except for extreme high and low-temperature characteristics, these hydrocracked oil formulations are the practical equivalent of PAOs in most automotive and industrial lubricant applications.
The American Petroleum Institute (API) developed a classification or grouping of lubricant base stocks to better show the differences between the different types. Each type of lubricant base stock is included in one of five groups as summarized below:
| Group | Saturate Level (wt. %) | Sulfur Content (wt. %) | Viscosity Index (VI) | Comment |
| I | < 90 | > 0.03 | 80 ≤ VI < 120 | Conventional Petroleum Oil |
| II | ≥ 90 | < 0.03 | 80 ≤ VI < 120 | Hydrocracked |
| III | ≥ 90 | < 0.03 | ≥ 120 | Hydrocracked and isomerized |
| IV | Polyalphaolefins (PAOs) | |||
| V | All other stocks not included in Group I, II, III or IV. (Esters, PAGs, silicones, etc.) | |||
In the API classification, all the oils except Group I can legally be called “synthetic”, but there are different chemical properties and characteristics associated with each of Groups II through V.