Castrol Universal 75W-90 is a multi-purpose transmission oil, intended for use in synchronized manual transmissions, differentials and final drives, requiring compliance with API GL-4 or API GL-5 classifications. BENEFITS. One product for transmissions and final drives reduces storage costs and a risk of misapplication. Excellent thermal stability ensures excellent performance characteristics and enables to potentially extend the service intervals. Optimum operation conditions of synchronizers ensure smooth gear shifting in a wide temperature range. Excellent protection from wear ensures the extended service life of the transmission parts and reduced maintenance costs.