Temperature

• The correct choice of transmission oil depends on its viscosity at the minimum and maximum temperatures at which the transmission is expected to operate. You should also take the permissible range of viscosities for the individual components of the transmission into account.
  
  

Viscosity Number

• The Society of Automotive Engineers (SAE) defines a series of viscosity numbers, which describe the viscosity, or thickness, of transmission oils at various temperatures. It is important to note that the system for transmission oil is different from that for regular engine oil.
  
  

Trends

• High-viscosity transmission oil creates drag on the gears, which creates heat, reduces power and increases fuel consumption. New transmission systems tend to use thinner transmission oil, but older systems may rely on thicker oil.