While the general mechanical design and function of synchronized manual transmissions has remained fairly constant over the years, incremental improvements of all components - gears, bearings, seals, synchronizers, and fluids - continue to advance the quality of the product. The improvements are generally driven by marketplace demands aimed at durability and shift quality. The synchronizer and fluid are two design components that can affect the overall performance of the transmission as observed by the end user.In recent years there has been a variety of synchronizer materials from brass and molybdenum based products to include Sinter compositions as well as phenolic materials-particularly in Japan. Each composition affords the designer different wear and durability properties. For example, although the Sinter is a Cu based alloy like the brass; fluid response is not necessarily the same. Thus, there is a need for revision of the fluid composition to obtain the optimum effect from the synchronizer material. This paper examines the effect of fluids on the friction and wear performance of synchronizer materials in carefully controlled laboratory tests. In addition, the correct test methodology allows one to develop information about the shift quality of the fluid-synchronizer material combination. To achieve maximum benefit from the lubricant-gearbox system it is essential that the rheological and chemical properties of the fluid are in balance with the operating environment, hence the inclusion of sections on oxidation and seals.