People run all sorts of oils in them without problems including generic "tractor hydraulic fluid" but the consensus is that it's probably best to use what the manufacturer recommends.
Before you complain too much about the cost I can tell you that Rolls Royce also specifies that their cars use a special mineral based hydraulic oil in their systems and that stuff costs $25 a liter, which is at least double what the febi stuff costs. Jaguar also uses special stuff which costs $20 a liter. It's just the cost of owning some "special" cars
