This is what Eaton says:
Fluid Selection
Premium grade petroleum based hydraulic fluidswill provide the
best performance in Eaton hydraulic components. These fluids
typically contain additives that are beneficialtohydraulic systems.
Eaton recommends fluids that contain anti-wear agents, rust
inhibitors, anti-foamingagents, andoxidationinhibitors.Premium
grade petroleum based hydraulic fluids carry an ISO VG rating.
SAE grade crankcase oils may be used in systems that employ
Eaton hydraulic components, but it should be noted that these
oils may not contain all of the recommended additives. This
means using crankcase oils may increase fluid maintenance
requirements.
Hydraulic fluids that contain V.I. (viscosity index) improvers,
sometimes called multi-viscosity oils, may be used in systems
that employ Eaton hydraulic components. These V.I. improved
fluids are known to “shear-down” with use. This means that their
actual viscosity drops below the rated value. Fluid maintenance
must be increased if V.I. improved fluids are used. Automotive
automatic transmission fluids contain V.I. improvers.
Synthetic fluids may be used in Eaton hydraulic components. A
reputable fluid supplier can provide information on synthetic
fluids. Reviewapplications thatrequire the use of synthetic fluids
with your Eaton representative.