ISO AW 100. Scroll down on the right to see the specs. This is a custom formulated synthetic hydraulic oil. My guess it's out of our pocket books.
I use a multi-grade hydraulic oil AW32-46-68 with excellent results in all weather conditions (in Wash. state). I wouldn't use UTF mainly because you are throwing money away, and it is not specified for straight hydraulic systems. You will be fine with ISO AW 32 or ISO AW 46 hydraulic oils. Try not to use straight AW machine oil, and especially engine oil