Is the hydraulic system for the cylinder isolated from the drive line? When I worked in a steel mill as an equipment mechanic, almost all our lift hydraulic systems were isolated from the drive line and we used 30HD motor oil in them.
If your lift system is not isolated AND you have wet clutches, etc, then you have to use what the OEM specifies or it"s equivalent.
In TO-20 and TO-30 Fergusons where you have no wet clutches, several people have used motor oils in those transhydraulic systems(again, no wet clutches) and have reported no adverse effects. The spec oil is GL-1 and it"s getting hard to find or expensive. In England apparently GL-1 is unavailable and one forum member contacted Castrol and they recommended motor oil as a substitute.
Hope this helps you.