Using the recently developed hydrostatic disc machine, a new form of gear lubricant failure has been investigated. This is called fatigue scoring and is caused by microcracks and micropits forming in the surface of heavily loaded surfaces, which drain away the oil in the contact. It is shown that failure is viscosity sensitive: the lower the viscosity, the lower the resistance to scoring. The correct choice of EP additive can raise the scoring load considerably, providing the additive is so chosen that it is active at the running temperature of the gear surface.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE International Lubrication Conference held in New York City, October 9–12, 1972