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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.

Laboratory testing is an essential part of product development. However, it usually only reflects a small portion of the experience that a lubricant may see in actual service conditions. Many laboratory tests are designed to only address one or two facets of what is deemed to be critical performance areas. Since it is difficult to cover all of the critical performance conditions problems sometimes arise in service that were not anticipated by the laboratory test. Or, conversely, some above average performance evolves during service that was not observed in a specific laboratory test. This paper highlights the overall performance of four manual transmission fluids approved or accepted by the manufacturer for this application. The evaluations were conducted in a city bus fleet with the test buses assigned to the same route for approximately 300,000 km over 30 months.