I was reading an article about keeping the large machines running in the Arctic and Antarctic, and it had to do with a central heater, and a manifold assembly. When the equipment is finished for the day, they either bring them inside a bldg, or leave them outside, and they use QD to plug in for the antifreeze, and engine oil, and hydraulics. The fluid is circulated constantly, and heated, and when they are ready to start a new day, the machine is warmed up enough to go out and operate without worrying about cold start ups, etc.
You could do the same for your hydraulic machine, by plugging into a hydraulic manifold and heating system in your garage or barn, that keeps the hyd fluid circulating at a certain temp, and filtering the fluid at the same time.
A 5 gal container, an electric motor, small hyd pump, filter, and QD's.
When finished for the day, drive up to the heating station, plug in, set the timer, or temp setting and when you next want to use the machine, it will be warmed up and ready to go.