Raw crude oil is not a good lubricant. It contains high levels of nitrogen and sulphur compounds as well as wax crystals. Wax and sulphur cause base oil to be less stable, providing targets for heat and chemical degradation that shorten the oil’s useful lifespan.
Crude oil is constructed from a wide variety of long chain hydrocarbon molecules which allow oxygen to attack gaps in the molecular structure when the oil gets hot.
When crude oil is refined into base oil, molecules are separated according to their weight which has the effect of separating unwanted materials from desirable ones.
Three hydro-processing methods are used in ultra refining the oil to further enhance its desirable properties.
Hydrotreating is a process of adding hydrogen to the base oil at elevated temperatures in the presence of catalyst. They attach themselves to “gaps” in the oil’s molecular structure blocking oxygen molecules form oxidizing the oil, increasing the useful life of the base oil.
Hydrotreating removes some of the nitrogen and sulphur molecules from the base oil.
Hydro-cracking is a more severe form of hydro-processing. It is done by adding hydrogen to the base oil feed at even higher temperatures and pressures. Oil molecules are reshaped and cracked into smaller molecules leaving fewer gaps for oxygen molecules to attach themselves.
A great majority of the sulphur, nitrogen compounds, and aromatics are removed.
Wax hydro-isomerisation is a third step, which lowers the pour point of the base oil so that it flows well at low temperatures. Wax hydro-isomerisation also removes the majority of remaining sulphur and nitrogen making a base oil with exceptional purity and stability. The oil, typically, has no color.
Waxes are removed from the oil so it does not freeze in cold temperatures.
Aromatics are removed as they increase the tendency of the oil to oxidize and thicken at high temperatures reducing the oil’s viscosity index.
Lubricants are designed so their viscosity is low enough for good cold weather starting, yet high enough to provide adequate film thickness and lubricity at high engine temperatures. The oil’s viscosity has to remain consistent when the engine is hot or when operating in a cold environment. The oil’s ability to cope with this is the viscosity index. A high Viscosity index indicates a small change over the oil’s operating temperature range. A low viscosity index indicates the oil has a poor response to temperature change.
Group I base oil is characterized as having greater than 10% aromatics and greater than 300 parts per million sulphur content. The viscosity index of Group 1 base oil is less than 80.
Group II base oils contain significantly lower levels of impurities than Group I base oils - less than 10% aromatics and less than 300 parts per million Sulphur content, however the viscosity index of Group I1 base oil is also less than 80.
Group II oils are more inert than Group 1 base oil and forms less oxidation by-products. Improved purity means that the base oil and the additives in the finished product can last much longer. In some cases, lubricating oils formulated with Group II base oils can outlive more expensive synthetic oils.
Group III base oils are manufactured by essentially the same processing method as modern Group II base oils. They have a higher viscosity index, greater than 120, than Group II base oils which is achieved by increasing the temperature or time in the hydro-cracker.
Modern Group III base oils have properties which allow them to perform at a level that is significantly higher than Group I and Group II base oils. They can match the performance levels of synthetic oils.
Group IV or traditional “Synthetic” base oils are chemically engineered base oils. Man made Hydrocarbons called Poly-alpha-olefins or PAO's are a common example of a synthetic base stock.
Synthetic base oil can also be made by removing molecules from Ethylene gas, that accompanies crude as it comes from the ground, and reconstructing them to produce high quality base oil.
When combined with additives, synthetic base oils offer excellent lubrication properties. They are chemically stable and have uniform molecular chains.
Oxidation resistance and thermal stability are are also very high when compared to Group I, II and III base oils.
Better base oil stability means better additive stability and longer life with longer drain intervals.
In the past Group IV/PAO base oils have been superior to Group III base oils, particularly in regard to viscosity index, pour point, volatility, and oxidation stability. However, modern Group III oils can be designed and manufactured so that their performance closely matches Group IV/PAO base oils in most applications.
For this reason they may be legally labeled synthetic, as they have most of the performance characteristics of early synthetic base oils.
All the base oil groups: As defined by the American Petroleum Institute (API) Publication 1509.
Group Sulfur, Wt % Saturates V.I. I >0.03 and/or <90 80-119 II ≤0.03 and ≥90 80-119 III ≤0.03 and ≥90 ≥120 IV All Polyalphaolefins (PAOs)
Source: CDX Global
