Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in
automobiles, trucks, and other machinery. It is of a higher viscosity to better protect the gears and
usually is associated with a strong sulfur smell. The high viscosity ensures transfer of lubricant throughout
the gear train.
This is necessary since the devices needing this heavy oil do not have pumps for transferring the oil
with only a portion of the lowermost gears bathed in an oil sump.