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Over the years a wide variety of lubricant types have been used for manual transmissions. Within the last decade, changes in vehicle design, operation and loading factors have led to the development of dedicated fluids to ensure optimized transmission operation. This paper reviews these changes and outlines the market forces that have necessitated the current commercial vehicle heavy-duty manual transmission fluid specifications and categories.

The authors provide evidence indicating that oils meeting only the minimum requirements of API GL-5 do not always provide adequate gear protection, especially in severe duty applications. Increases in commercial vehicle power and loading have accentuated the need for oils of greater load carrying ability. A modified version of the standard L-37 test may help identify oils that possess superior durability and thermal characteristics. Future gear lubricants should provide improved fuel economy, increased manual transmission life; and frictional characteristics that allow noise free performance in limited slip differentials.