While a high base oil VI may require less VI Improver, there is also oxidative stability to consider. In the hydrocarbon family, VI correlates well to oxidative stability since the less stable components tend to have lower VIs. By removing or restructuring these less stable components the VI increases. This is what hydrocracking achieves.
The most oxidatively stable components from crude oil derivatives are saturated linear hydrocarbons, but these waxy chemicals exhibit a freeze point upon cooling and are therefore undesirable and mostly removed. The next best for oxidative stability is saturated branched hydrocarbons as the branching eliminates the freeze point tendency, thus allowing a true pour point. PAOs are mixtures of saturated branched hydrocarbons and have the highest oxidative stability. Group III base oils are next as hydrocracking restructures the low VI less stable molecules, mostly into branched molecules.
The same correlation of VI to oxidative stability is not necessarily seen in other chemical families. For example, many vegetable based esters have very high VIs, up to 200+, yet are less stable than Group I hydrocarbons due to unsaturation. And the PAG family can also have very high VIs yet are less stable than POEs and diesters with lower VIs.
Keep in mind, however, that these differences in oxidative stability can be overwhelmed by a good anti-oxidant package, so it is once again the finished formulation that actually dictates the performance of the oil.
Tom NJ/VA