AbstractExperience with existing oils used in helicopters for transmissions, engines, hydraulics and brakes has shown that three main characteristics could drastically improve the performance of the aircraft.These characteristics are: (1) the same type of oil should be usable for all the above mentioned systems; (2) the oil brands, produced to a common specification, but by different manufacturers, should be mixable without restrictions; (3) some features of the oil, which are specially important for helicopter operations, should be improved. Tests on mixed oils are reported, and recommendations made for future specifications.