Base oils are the main constituent of any engine oil and the quality of the base oil used plays a significant role in how well the finished product performs. As we have detailed elsewhere, however, it is not the base oil alone that decides if a finished oil is suitable for an application or that it will fulfil the manufacturer’s specifications. Base oils are categorised by the American Petroleum Institute or API, and split up into five main groups. Figure 1 shows how the basic oils are produced.
Table 1 shows the API grouping of the base oils. The table also gives an overview of how oils are created for a specific viscosity and a general indication of the costs associated with differing oils. The higher cost of the synthetic oils is mostly due to the much higher complexity and cost of production.
Table 1 is an approximate guide as to which oil viscosities belong in which group and also shows that there is significant overlap between group III hydrocracked oils and group IV oils. The table also clearly illustrates that oils using less refined mineral base oils only have a limited range of applications as they cannot meets the demands of the modern engines.
Base oils and factors affecting lubricant selection
Table 2 gives an overview of oil’s characteristics in each group and can used as a basis for a decision on which oil type to select. As you can see, the decision is not as binary as many would have you believe i.e., either mineral or synthetic. Modern mineral-based oils are best suited for higher viscosity ranges, in engines that put the oil under less stress and where emissions requirements are less stringent.
Furthermore, mineral oils are recommended for applications where a synthetic oil is neither prescribed by the vehicle manufacturer, nor suitable. In fact, using an expensive modern synthetic oil in an engine that was designed to use a higher viscosity, mineral-based oil can be counterproductive.
If the vehicle manufacturer specifies a low viscosity oil, hydrocracked oils are often the best choice. Modern hydrocracked (group III) oils can actually outperform a PAO in several areas, such as additive solubility, lubricity and anti-wear performance. The advances in technology, mean that individual characteristics can be controlled in a much more targeted way. The precise management of characteristics allows lubricant manufacturers to formulate an HC oil that at least matches the performance levels of considerably more expensive fully-synthetic oils, whilst keeping the amount of synthetic oil added to an absolute minimum. Only in the very latest engines – where ultra-low viscosity oils may be required to meet the most stringent specifications and demands – are fully synthetic oils strictly necessary.