Bret there is not much that you can safely use to thin that hydraulic oil. You could use some diesel fuel and that would work for a while but even in cold weather the oil will warm up and it then will be too thin to lubricate the pump and the valves in the controls. If the tractor starts in this minus 20 weather it is best to let run for a while before using the hydraulics. With my 8 year old Kubota tractor, that I keep in a shed, I let it run for 20 minutes or longer when the temp is below 10 degrees and so far no problems.
If you do use something to dilute the hydraulic oil remember to drain it when warmer weather comes. Repairs or replacement of a hydraulic pump can take a big bite out of that state pension! Happy cold weather farming and happy new year.