Just to chime in a little late.
I have owned a T9 for the better part of 10 years. It sits outside in Missouri 365 days a year. When I got it, I replaced the oil in both the engine and the hydraulic system. The oil I use in the hydraulics is an inexpensive Walmart 15W40. With some wear on the seals causing a little leakage, I just top it up from time to time with the same stuff. Never changed it again. I use it to push snow off the drive and have never had an issue with the hydraulics not working, even below zero. Of course, getting that little Kubota started when it is below freezing requires my 300amp charger and really getting the glow plugs warmed up, but once it is going, a 5 minute warmup and everything is perfect.
Unless you are using the T9 over the winter in constant below freezing temperatures, I would not go through the trouble and expense of changing the oil in the hydraulic system. When Terramite was really in business, this is what I was told by their tech.