Barry,
If you are considering the Amsoil hydraulic oil
{HERE} you should keep in mind that it is currently designated as a multi-viscosity 5W30 weight oil. Formerly, it was designated as straight SAE 30/ISO 46 oil. ISO 46 is roughly a 20W oil, not 30, and maybe that's why Amsoil changed their lower viscosity limit and re-named the oil 5W30, eliminating the ISO designation altogether. Bottom line, Amsoil hydraulic oil is 5W30 oil.
IMO, in the southeast, it just doesn't get cold enough to warrant such a thin oil in the hydraulics. Remember that PT recommends 15W40 motor oil for the hydraulics. Many folks on the forum have reported better performance under stress, e.g., hill climbing, etc. when using even a higher viscosity motor oil in the hydraulic system, such as 15/50 or 20/50. A 15 weight oil should pose no problems with cold weather starts here, and the 40-50 weight upper spec will give you hydraulics the oomph under heat/stress that folks have reported here.
If you are considering a true hydraulic oil (as opposed to motor oils used in a hydraulic application) for your system, the 15/40 would roughly correspond roughly to an ISO viscosity number of 60-100
{HERE} . Amsoil does make a synthetic anti-wear hydraulic oil with an ISO viscosity of 68
{HERE}.