Helicopter manufacturers' requirements for lubricating oils

Experience 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.