What may not be appreciated is that the VI of a base oil tells us more than just viscosity-temperature profiles. But first it is worth appreciating what it is about a hydrocarbon molecule that determines its contribution to the VI of a base stock. A base stock is essentially a soup of many different chemical types of hydrocarbon: paraffins, naphthenics and aromatics, each making their own VI contributions.

One significant contributor to the overall VI of a base stock is the effective molecular size of an individual hydrocarbon type. In its simplest form, size can be expressed in terms of the carbon number, that is, the total number of carbon atoms in a given molecular type. Molecules with high carbon numbers make high contributions to VI and vice versa.

This so-called grade contribution is the prime reason viscosity index improvers (VIIs), usually hydrocarbon polymers, are so effective in raising the VI of the lube – they simply have extremely high average carbon numbers. There is a popular misconception that hydrocarbon VIIs work because they expand as lubricant temperature increases, but in fact it is because they are always larger than base oil molecules whose VI they boost. Whether viscosity modifiers expand or contract as a function of temperature (some do expand, most dont) is largely irrelevant.

The effect of molecular size applies to species that are of the same chemistry type. The role of size means that as the viscosity grade increases, the viscosity index increases. This is most clearly seen in a grade slate of polyalphaolefins, where the basic chemistry is a constant set of iso-paraffins, but the heavier grades have higher VI than the lighter grades by some 20 points.

Another example may be seen in API Group III base oil production. The light fraction between gas oil and conventional lube base oil grades, say 2 to 3 cSt, often fall below the 120 VI threshold – for no other reason than they have low average carbon numbers – and there is no process way to change this. For Group I, Group II and Group III base stocks that are crude-derived it is often not so easy to see this simple grade effect on VI, since increasing viscosity grade is often accompanied by a mix of off-setting high and low VI molecular types.

We find that n-paraffins (non-branched straight chains) have the greatest contribution to the VI of a base stock among hydrocarbons since compared, for example, to iso-paraffins (branched paraffins) they have a larger effective molecular volume. This is because all the carbon in the n-paraffin is in the main chain and none in the side chain. As we go to naphthenic and then aromatic molecules, we find a sequentially lower contribution to the overall VI of a base stock because their ring-type structures make the molecules much more condensed or contracted, such that the molecule has a lower effective volume for a given carbon number.

Molecular size however, whilst a major factor, is not the only factor contributing to the VI of a base stock. It will not explain why, for example, a mono-aromatic molecule has a lower VI contribution than a mono-naphthenic molecule of the same carbon number, although both molecules have a single six-membered ring and the same length of side chain.

One thing VI cannot tell us, ironically, is the precise low-temperature performance of a base stock, say at minus 35 or minus 40 degrees C, even though many assume very high VI base stocks will always have the best low-temperature performance. This is because VI is a parameter which discounts any contribution from effects such as residual wax crystallization, or even the molecular associations that precede wax crystallization, which can contribute to increases in viscosity as temperature drops. In fact, we find that some of the higher VI base stocks have worse extreme low-temperature performance, just because the VI value may be boosted by molecules with a lot of n-paraffinic or waxy character.

Consider some practical examples – the low-temperature performance of base stocks as required for the SAE 0W-XX cold crank viscosity or automatic transmission fluids Brookfield viscosity. We indeed see is that there is poor correlation between base stock VI and the low viscosities required for cold cranking at say minus 35 degrees C. PAOs with VI of around 120 can often function here relatively better than higher VI Group III base oils.

So VI will not necessarily be a good indicator of extreme low-temperature response, but it will be a very good indicator of retained viscosity and hence higher oil film thickness at the higher-temperature operating conditions of finished lubricants, because all waxing effects will have melted and gone, and we will just be dealing with a simpler homogenous fluid.

VI is an extremely good indicator of the compositional quality of a base stock since the highest VI components, as well as having the highest boiling points for a given carbon number, also generally contribute far better to the antioxidant additive response, for well understood reasons.

So in summary, VI will tell us about a base stocks high-temperature viscosity, boiling point characteristics and be indicative of antioxidant response. What it will not reliably tell us is how hydrocarbon base stocks perform viscometrically at extremes of low temperature.