AbstractWithin the framework of the high safety requirement imposed on aero engines, it is understandable that the oil for these engines should also be subject to high quality requirements. These have risen over the years alongside engine performances up to supersonic flight, where mineral oils could no longer meet the demands imposed by such high‐performance engines. They have been superseded by synthetic ester oils, especially because of their more suitable viscosity‐temperature behaviour. This paper deals with the behaviour of these synthetic oils in aero engines and concentrates on the thermo‐oxidative strength and the effects on the engine oil and the engine i