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One very effective way to improve fuel economy of motor vehicles is to reduce the viscosity of the automatic transmission fluid (ATF). However, viscometric constraints often exist for a particular ATF application. The proper choice of viscosity index (VI) improver allows viscosity reduction at lower temperatures, while maintaining viscosity at higher temperatures. This study shows that molecular weight of a VI improver is a key factor to achieve high thickening efficiency, improved shear stability and limited metal fatigue failure simultaneously. Based on this study, an optimum VI improver has been found and a new fuel economy ATF was developed.

Torque loss reduction at differential gear unit is important to improve the fuel economy of automobiles. One effective way is to decrease the viscosity of lubricants as it results in less churning loss. However, this option creates a higher potential for thin oil films, which could damage the mechanical parts. At tapered roller bearings, in particular, wear at the large end face of rollers and its counterpart, known as bearing bottom wear is one of major failure modes. To understand the wear mechanism, wear at the rolling contact surface of rollers and its counterpart, known as bearing side wear, was also observed to confirm the wear impact on the tapered roller bearings. Because gear oils are also required to avoid seizure under extreme pressure, the combination of a phosphorus anti-wear agent and a sulfurous extreme pressure agent are formulated.