The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the high polymer content. Very few manufactures recommend 10W-40 any more, and some threaten to void warranties if it is used. It was not included in this article for that reason. Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle. Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range.
Higher numbers indicate a low change, lower numbers indicate a relatively large change.
The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown. Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil.