Alternative to hydraulic oil in air over oil system

a aw32 oil suggests a thin (32 mm2/s) oil that contains some form of antiwear additive, usually zinc based. that could also be considered to be a low viscous hydraulic oil...hydraulic oils are usually formulated in such a way that they have good air release properties - you do not want air bubbles entrained in a pressurized fluid, whereas a small amount of foam on top of the fluid in a nonpressurized reservoir does not pose a problem as long as the pump is fed from a region within the reservoir where no foam is available. foaming within hydraulic systems most times is caused by either dirt that kills the air release properties of the fluid or by design faults in the hydraulic system ("splashy" return line or to a too small volume of the reservoir, so no complete air release is possible. in theory the foaming tendency can be decreased by adding a small amount of antifoaming agent (a few ppm!) but usually that increases the air release time quite a bit and thus causing a bigger problem then the problem solved....

steam turbine oils are known for their excellent air release properties and very low foaming tendency. thus, a ISO VG 32 steam turbine oil might be a alternative, as long as no antiwear properties are expressly required.