API base oil groups are a set of specifications to discriminate between significantly different base oil types. These specifications were formulated in the mid-1990s in response to the different base oil compositions that were appearing in the market at that time, especially as severe hydroprocessing became more widespread and provided an alternative manufacturing approach to classic extraction refining.
Since that time, the five API Groups have served the lubricant industry well. The specification set is effectively global, since Atiel (the Technical Association of the European Lubricants Industry) aligned with the same groups for base oil definition and interchange for ACEA lubricant products. Atiel looks after base oil and lubricants matters for ACEA, the European Automobile Manufacturers Association. Atiel also had a short-term excursion with a sixth group to allow conditional interchange between Group VI poly internal olefins and Group IV polyalphaolefins, now discontinued since there are no current manufacturers of lubricant PIOs.
With API base oil groups came the definitions of base oil slates – sets of base stock grades from a given API group made by a single manufacturer. As long as certain criteria are met, oils in a base stock slate are fully interchangeable for blending purposes, even if they come from different production plants. This is all described in Appendix E of API 1509 Engine Oil Licensing and Certification System or the Atiel Code of Practise, Appendix B.
Are the current API/Atiel groupings still adequate for todays base stock market? Judging by the discussions that have taken place within relevant API and Atiel lubes committees over the last couple of years, some would suggest no. So what are the issues?
Some have argued that there is a very wide range of base oil compositions and qualities that are not adequately described or differentiated by the current group criteria of saturates, sulfur and viscosity index, so maybe subdivisions are necessary and/or alternate ways of describing them. I personally would argue that such a range of compositions even within Group II and III, has always been there. For example, extremely high VI base stocks from very waxy feeds, unofficially called Group III+, have been with us since before the inception of the API Group system. Also a large compositional span has existed within Group II, either due to the higher VI Group II+ or those Group IIs that do not have the very high saturates content (approximately 99 percent by weight) typical of severe hydrotreatment.
Has this resulted in field problems? Not to my knowledge, since good product developers know what base stocks they are dealing with and formulate using so-called worst case development.
Should there be a more detailed specification set based on some new set of chemical descriptors, and what should those? Modern analytical techniques certainly allow a much more detailed characterization of base stocks than was routinely available in the mid-1990s – but any new approach in characterization would have to be generally available to refiners and purchasers, and availability could be problematic. Also, attempting to second guess all the various base oil responses in engine and bench tests through more detailed definitions of base stock types likely wont work or even improve interchangeability.
Another issue is that whilst the group definitions cover very wide compositional ranges, some groups, especially Group II and Group III, the typical properties of the average base stock offerings on the market today are largely bunched at extremes of the specification set. For example, the majority of Group II base oils, especially lighter grades, will have typical saturates levels above 99 percent by weight, yet the specification requires more than 90 percent saturates. Also sulfur contents of severely hydroprocessed Group II and Group III are almost always less than 5 parts per million, nowhere near the specification limit of 300 ppm, and whilst Group I and Group II have a VI specification range between 80 and 119, VIs of the bulk of high viscosity index base oils are between 95 and 119. In fact, Group I and II base oils with VI of 80 would almost certainly be unusable for lubricant applications for a variety of reasons – including probable health, safety and environmental limitations in the case of Group I.
So do specifications need to be redrawn? Well certainly we could say that base stocks from the compositional extremes of any group will have vastly different additive responses. Just because a base stock sits within the current specification set does not mean it is equivalent, if it is at the opposite of the specification set. This is the principal reason we have base oil interchange testing.
A further issue that has arisen within API is what constitutes an interchangeable base oil slate. It is now obvious that some of the definitions for API Groups and interchangeable base stock slates as described in API 1509, have been interpreted differently by individuals. This has caused much debate.
So it is quite likely that API 1509 will at the very least get some updates to remove such ambiguities in due course. Whether we see any fundamental change to the number of groups or their specifications is another matter and will probably receive quite a lot more debate in the relevant API and Atiel committees.