Viscosity Index is a direct mathematical function of the KV40 and KV 100 numbers. The larger the difference between KV40 and KV100, the larger the VI. If you start with a low VI base stock, Viscosity Index Improver polymer can be added to get a larger difference between KV40 and KV100, thereby increasing the viscosity index. Typically, VII is added to a low-viscosity base stock to increase the KV40 number, then at high temperature the polymer chains in the VII uncoil, decreasing the KV100 number.
The trouble with VII's is when they are subjected to shear within a bearing (or between any two moving surfaces), their viscosity breaks down, yielding lower film thickness than if another oil of the same kinematic viscosities, but no VII content were used. This is a temporary loss of viscosity that only exists when the oil is in the bearing.
Another phenomenon that occurs with oil containing VII is that as it repeatedly runs through the engine, the polymer's chains get broken, resulting in a permanent viscosity loss. Depending on the magnitude, this can cause the oil to permanently shear down in grade.
This is where certain oils, like Redline 5w30 stand out: the POE base stocks have an inherently high VI without requiring VII's, so they are more resistant to shearing down in grade and have higher HTHS viscosities in relation to their kinematic viscosities.
How high of a Viscosity Index do you need? It depends on what you want the oil to do. If you want to run the same oil year round, and are driving between Key West and Anchorage, you would want an oil with high VI to get maximum protection over a wide range of temperatures. If you are running an alcohol sprint car during the summer only, you would want to run a straight-weight (low VI) oil to get maximum protection from shearing, and then dump the oil after every race.
