Since gear and transmission oils are subjected to widely varying temperatures, particularly in vehicles, it is desirable that such lubricants have as little change in viscosity with temperature changes as is possible. In other words, high V.I. oils are desirable in many gear oil applications and absolutely necessary in certain cases. An illustration of the latter is automatic transmission fluid. Base oils with V.I. values of 90 to 95 are readily available when needed. Where it is desired to increase the V.I. of oil, additives, known as V.I. improvers, are sometimes used. These agents are generally polymers which act by either coiling up or becoming less soluble at low temperatures but reverse this action as temperatures increase, so as to contribute increasing body at higher temperatures. In normal gear operation the shearing effect tends to break down such polymers into those of lower molecular weight which are less effective as V.I. improvers. However, these agents do have a place in services such as ATF where they maintain their effectiveness.
Certain high molecular weight petroleum resins have been found to improve the V.I. of some lubricating oils and not have the defect of breakdown with shear.
