Initially I was going to say any 100wt hydraulic oil would do, but having found a spec sheet, it looks like Omala has EP additives to make it more gearbox friendly. The most common EP additive used to be sulphur, however the spec states that Omala is bronze friendly, so it doesn't contain sulphur (sulphur attacks/disolves bronze).


Personally, if it was my own machine, and I wasn't running it at high loads all the time, I'd stick some ISO100 hydraulic oil in, as lots of machines have specified and used normal hydraulic oil for many years (usually under the guise of Shell Tellus!)
Other option is to use some of the more modern car gearbox oils, as they're pretty thin due to the manufacturers doing everything possible to improve effiency.