I'm not a petro engineer, so I guess I'm using metaphors to sort out this subject. Something like thicker & stickier equals better lubrication, provided it gets to all parts. Then I realized, that I don't have any idea what the properties of hydraulic and gear oils would be at the points of gear contact. The might be the same for all I know. There certainly are huge pressures--enough to weld the surfaces of the gear faces together if overloaded.
In terms of the equivalence of hydraulic and gear oils: It's possible that they aren't equivalent, but they might give similar expected transmission lives. The idea is that the components inside a gearbox wear for different reasons and probably have different lubrication requirements. No single lubricant is likely to be ideal for all components. Perhaps, some components tend to fail when gear oil is used, and other components fail with hydraulic oil, but the expected lives are similar.
Another possibility is that the subject received little engineering attention. Maybe something fairly unrelated to lubrication (shift collars?) are expected to fail first. An assumption is that a mechanic will be into a case before a rotating component is expected to fail and will replace things as needed before failure.
Of course, these are just speculative ideas.
