The evolution of hydraulic fluids has paralleled the history of aviation. In early years the hydraulic systems were required only to apply brake pressure. These systems used a vegetable oil or castor oil based fluid and natural rubber for seals and hoses. As aircraft designers produced larger and faster aircraft, the role of hydraulics expanded, becoming a key component in operating landing gears and flight controls.These applications resulted in higher pressures (1000 psi) and called for improved fluids. To meet these demands the aircraft industry turned to petroleum based fluids. Since the petroleum oils caused the natural rubber to swell, the aircraft designers switched over to synthetic rubbers such as Buna N or Neoprene.

Further advances in aircraft required hydraulic systems to work at even higher pressures (3000 psi) and extremes of temperatures (-50oC to + 120oC). This led to the development of very high viscosity index fluids which are still in use.

The petroleum based fluids are inherently flammable and their use in commercial aircraft has been withdrawn, where the synthetic non-flammable phosphate ester fluids are being used. Even for military aircraft, products based on synthetic basestocks have been developed The major hydraulic fluid specifications and product designations are:

 

AIRCRAFT HYDRAULIC FLUIDS TYPE
SPECIFICATION	JSD	NATO CODE	TYPE
MIL-PRF-5606H DEF STAN 91-48/2 DCSEA 415/A (AIR 3520/B)	OM-15	H-515	Mineral
MIL-PRF-83282D	OX-19	H-537	Synthetic
MIL-PRF-87257B	OX-538	H-538	Synthetic
GOST 6794-75	AMG-10	-	Mineral
GOST 20734-75	7-50S-3	-	Synthetic