Why would you risk doing hundreds to thousands of dollars damage for the sake of $12 worth of oil? Hyd jacks have no high speed moving parts, and a system that generates no heat, it is probly one of the lowest grade oil they can make, it's only requirement is that it thin, so it flow iin a jack that never sees "operating temperature" KenI would say no, hydraulic jack oil is too thin. You need 10w30 or 10w40 oil, like used in car engines. If its too thin, it will damage your hydrostatic system.
I wouldn't risk it, buy the correct fluid.