SAE J300 for crankcase finished lubes lists the range of kinematic viscosity at 100 degrees C for a given grade. It does not specify how that range should be achieved in terms of base stocks, performance additives and viscosity modifiers, all of which make viscosity contributions.

Base oil viscosity contribution usually is mentioned only in some base oil interchange tables. But when formulating a crankcase lube, kinematic viscosity gets significant contributions from the additive package and viscosity modifier (for multigrades) in addition to the base oil.

Crankcase additive packages normally contain ashless dispersants that have molecular weights far greater than any normal base stock component. Hence, the additives contribute thickening power and, indeed, make a significant contribution to the formulations final viscosity index. And viscosity modifiers are orders of magnitude larger than base stock molecules – again the source of their thickening power and VI boost to formulations.

But returning to the base oil contribution to kinematic viscosity, we realize that a surprisingly narrow range of base oil viscosities is required to formulate the majority of current multigrades – SAE 0W-20 to 15W-40, or even SAE 16. Around 4 to just under 8 centistokes will cover them for most additive package and viscosity modifier treats.

This certainly has consequences for setting up the production slate of a new base oil plant or even in modifying the grade structure of an existing plant. Put simply, for most modern multigrade applications, using say high viscosity index base stocks, either Group I or Group II, a base oil viscosity corresponding to a Solvent Neutral 350 is probably as heavy as we need.

A viscosity corresponding to SN-350 can be produced by blending a dumbbell of SN-150 and SN-500 – or any other combination either side of SN-350. But when setting up production volumes in a high-VI plant, all you need is to focus on is something around SN-350 along with significantly smaller volumes either side of this grade. Some enlightened base stock manufacturers follow this model already.

This grade structure is fine for multigrade blending, but what about monogrades? These grades generally have lower additive concentrations and a total absence of viscosity modifier. The lower thickening contribution from the non-base oil components throws a greater burden on the base stock.

The majority of high-speed crankcase monogrades today can be covered by SAE 30 and SAE 40, and a 100 degree C kinematic viscosity of 10 to 14 cSt will cover base oil viscosity requirements. Since 12 cSt corresponds to SN-600, the heaviest conventional distillate base stock grade, SAE 40, will need a touch of thicker base stock – that is, bright stock. Demand for such monogrades is still surprisingly high in some developing economies, and there are no obvious signs that demand will end any time soon.

Where there is demand for SAE 50 high-speed crankcase monogrades, the demand for bright stock is higher. Indeed, demand is greater as a percentage of the formulation in high-speed applications than in marine SAE 50 monogrades. This is because marine formulations benefit from much higher additive treat rates due to poor fuels. Hence, the non-base stock components add thickening power.

Bright stock conventionally has a 100 degree C kinematic viscosity of about 32 cSt. This is largely a result of cutting strategy of the lighter boiling distillate base oil grades and the separation cut. But 32 cSt is not required for any crankcase grades or most industrial lube grades; however, it is used in large proportions or almost neat for some industrial gear oils.

Normally, bright stock has a 40 degree C kinematic viscosity of around 460 cSt – corresponding to ISO 460 grade in industrial lubes. But again, ISO 460 is normally a pretty small percentage of most industrial lube portfolios.

The base oil viscosity needs of most industrial lubricants can be addressed in a similar manner to crankcase multigrades. ISO 22 to ISO 68 covers a large percentage of most industrial portfolios. Despite the minimal contributions of the very low additive treats in hydraulic and industrial turbine oils, the bulk of requirements can still be met with an SN-350 type high-VI base stock (Group I or Group II) and a lighter grade.

Thus, heavy neutrals are not necessarily a technical must-have for most modern crankcase multigrade applications; rather, they can be treated as a trimming grade for the medium neutral. High-speed crankcase monogrades will likely require a bright stock type grade for some time to come. But the bright stock cut need not necessarily be 32 cSt, just because thats what came out of the refining process in the past.

Indeed, if we seek to replace Group I bright stock with catalytically dewaxed Group II or even Group III residual grades, we will certainly have to compromise on kinematic viscosity to get acceptable catalytic dewaxing capability. But thats another story.

Clarification – February’s Base Oil Report stated that Sepahan is the largest supplier of premium Group l base oils in the Middle East. This should have stated that Sepahan is one of the largest suppliers.