For industrial (nonaerospace) applications, hydraulic system pressures can be increased to the “high pressure” (2000-5000psi) or even the “very high pressure” range (5000-10,-000 psi) with fluids available currently. Fluid properties that must be considered for high pressure use are: lubricity, antiwear, bulk modulus, cleanliness, vaporization temperature, stability, and seal compatibility. For fire resistance, extreme low temperature and extreme high temperature synthetics are required.High quality, petroleum-base fluids have the greatest number of relative advantages for regular high pressure hydraulic service. Principal among these advantages are: lubricity, antiwear, cost, availability, ease of sealing, and stability.New synthetic fluids are available with unique properties for special applications. Costs relative to petroleum oils range from 5:1 up to over 1000:1.