The engine oil specifications of ILSAC GF-4 (implemented 2004) and GF-5 (expected 2010) have in common three important OEM needs: (1) improved fuel economy and its retention, (2) emissions system compatibility (related to phosphorus and sulfur content), and (3) improved oil robustness for extended oil life capability (e.g., reduced high temperature deposits and engine sludge, improved oxidative stability, and better valve train wear protection). For emission systems compatibility, there is a trend towards lower phosphorus and sulfur content in engine oils while maintaining high anti-oxidation and anti-wear performances. In this paper, the friction-reducing, antiwear and antioxidant properties of a sulfur-free and phosphorus-free molybdate ester compound as lubricant additive are discussed. The investigation showed that the molybdate ester enables good friction-reducing ability of the lubricant, and maintains low friction coefficients even after period of severe oil oxidation, which indicates the potential for excellent fuel economy retention in passenger car vehicles. For other organo-molybdenum additives, the investigation showed their friction-reducing ability decreased greatly after oxidation of the oil, although the performance appeared to be very good with the fresh oil before aging. In addition, the molybdate ester demonstrated a synergistic effect in anti-wear protection with zinc dialkyl dithiophosphate (ZDDP), a well-known commonly used additive in engine oils; this suggests that excellent wear protection is possible for an engine oil with reduced sulfur and phosphorus content. Moreover, the molybdate ester is also a strong synergist with arylamine antioxidants, which can retard its depletion in oxidized oil, extend its oxidative induction time, and reduce its high temperature deposit-forming tendency. Thus, this type of sulfur-free and phosphorus-free organo-molybdenum lubricant additive truly provides multifunctional performances, reducing friction, wear, and oxidation of an engine oil when combined with other suitable additives. Due to its phosphorus-free, sulfur-free and multifunctional properties, the molybdate ester compound can be extremely useful in formulating modern engine oils with low phosphorus and sulfur content.
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