Non-Synthetic Manual transmissions in heavy duty vehicles generally require specific fluids to alleviate the resistance and subsequent wear commonly encountered through the changing of gears. These fluids also serve to transfer power and act as a coolant.
Heavy Duty Manual Transmissions are often subject to high levels of friction and therefore require superior friction modifiers and other unique additives in their respective fluids in order to prevent wear of internal components. The lubricants needed to protect and seal transmission components are not clearly defined: some manual transmissions (i.e. according to OEM specifications) will call for a simple gear oil while others may specify the need for an automatic transmission fluid.
The optimal Manual Transmission Fluid or MTF is not always an obvious choice so be sure to consult OEM manuals and specifications before choosing any fluid.