In recent years, most new-build base oil projects have fallen into the API Group III category. These base fluids are also known as very high viscosity index oils, meaning they have viscosity indices of at least 120. Some have inferred that their ascendance means the end of high viscosity index base oils, including Group II. This is probably a premature conclusion since lubricant requirements will create at least a viscometric need for high viscosity index base oils probably for years to come. This is why we are hearing an-nouncements for some new Group II builds, for example Luberefs project in Yanbu al Bahr, Saudi Arabia, and Chevrons in Pascagoula, Mississippi, U.S.
High VI paraffinic base oils are defined as having VI greater than 80 but less than 120, a range that includes all Group II and Group I oils. High VI base fluids have been the mainstay of crankcase and industrial lube blending for decades. We can say with reasonable certainty there will be no more Group I builds. Group I solvent extraction technology is inherently a smaller run, batch-type process and constrains the types of crude feedstocks that can be used. But what of Group II?
In practice, Group I and Group II base oil grades have essentially identical viscometric blending characteristics, except that Group Is, because they include bright stocks, have a maximum kinematic viscosity of approximately 32 centiStokes at 100 degrees C, while Group IIs top out around 12 cSt. Group IIIs top out at approximately 8 cSt. Chemically, however, Group IIs are significantly different from Group I – having much higher levels of saturates (paraffinic plus naphthenic molecules) and virtually zero sulfur.
In recent years the chemical differences have become increasingly important with the development of automotive engine test sequences and specific engine tests that discriminate the soot-handling characteristics and the oxidation performance of base oils – both of which depend on the saturates content of base oils. Lately, exhaust after-treatment devices have created a need for formulations that have medium and low levels of SAPs (sulfated ash, phosphorous and sulfur) and hence require Group II or Group III base stocks.
The saturates appetite started in the United States, with the API heavy-duty diesel engine oil category CG-4 and its Mack T-8 soot dispersancy test. This appetite has now migrated to European crankcase lubricant sequences – one good example being the ACEA E9 heavy-duty category, which includes API soot-handling engine tests.
The foregoing leads us to conclude that any new high VI base oil project would necessarily be for a Group II base oil. Why, though, will we need high VI base oils at all in the future?
Why not just use the higher VI Group III base oils where high saturates or low sulfur are required? There are several reasons, and the first of these is cost. Raising VI to Group III levels requires more severe hydroprocessing, and this results in cracking feedstock to light ends and hence a loss in base oil yield. An alternative way to make Group III is to use very high VI hydro-wax feedstock or very waxy crudes, but these are more expensive or constraining.
Recognizing then that Group II base oils have inherently lower manufacturing and procurement costs relative to Group III, it makes sense, where possible, to maximize the Group II content of a formulation. Even for SAE 10W-40 engine oils, the majority of the base oil in a formulation with constraints on saturates or sulfur can still be Group II rather than Group III. Of course, a top-tier SAE 10W-40 would probably still use all Group III.
Another reason we need simple high VI base oils, is viscosity grade preference, especially customer demand for the SAE 15W-40 multigrade oils for heavy-duty diesel engines in both developing and mature markets. The practice of using this viscosity grade in heavy-duty engines has a traditional following, especially among owner-drivers in the U.S., and unless marketers can migrate customers to thinner multigrades, the demand will remain for some time to come. If we tried to formulate an SAE 15W-40 with an all-Group III base oil diet, wed probably fail, and it would likely come out as an SAE 10W-40. This is because the very high VI of a Group III drives down the cold crank viscosity to an extent that the finished lubricant must be declared a 10W-XX.
Also, monograde demand is still significant in applications such as heavy-duty diesel oils for developing markets, gas engines for developing and mature markets and slow-speed, two-stroke marine engines worldwide. Most of this large-volume business is currently formulated with Group I base oils, but as Group I availability decreases, Group II base oils will have to fill some of the gap. For most of the aforementioned applications, a jump to Group III base oils – even if it were cost effective – would be very difficult technically and in some cases impossible with prevailing maximum viscosity grades.
A final area in which technical drivers can favor Group II base over Group III is in industrial lubricants. These have in recent times been reformulated to exploit improvements in additive responses of high-saturates base oils – especially antioxidant response. However, with Group III base oils having such high VI relative to Group II, and hence low kinematic viscosity at 40 degrees C, formulators are usually limited to an upper ISO grade of 46 with the current Group III base oils, so ISO 68 and ISO 100 oils would logically be blended with Group II base oils.
In summary then, we can say that there is both a cost and technical basis for Group II base oils that Group III cannot service.