Gas Turbine engine manufacturers recommend an oil change if such properties as viscosity, acidity, etc, of the oil in use, exceed certain values. Since increases in the properties mentioned indicate the lubricant to be undergoing thermo-oxidative stress, the advice appears sound. It is known however, that serious deposition problems (coke formation), can occur without attendant increases in the bulk oil properties referred to above. In the light of this phenomenon, this paper attempts to show what work is being carried out to understand better what is happening to oil in service.
