Interesting thread. I'd like to add some points, however.
Considerations for hydraulic oil quality and standards are different than that of motor oil. The quality of the oil isn't as important as the conditioning of the oil. Higher quality filtration is required, and you will see longer life out of hydraulic equipment from switching from cheap 10 micron nominal filtration to 3 micron absolute filtration, than you will out of just choosing better oil.
It's a case where better oil will let you get by with a poorly designed hydraulic system longer than cheap oil. Hydraulic oil isn't supposed to get very hot, and systems aren't designed to run well when the oil is cold (which is why heaters and coolers are important). Actually, a properly designed hydraulic system shouldn't require a heater at all, but oil heaters may be required in some instances (or a heating circuit).
Also, monitoring of the hydraulic oil is important. There are electronic particle counters, temperature transducers, water saturation transducers, TAN and viscosity sensors. If you can monitor your oil effectively, it quite literally will never have to be changed (although additives can still be...uh...added)