My neighbor for one...It was 33F according the gauge with a wind chill of 25F. Actually it was leaves he was cutting up.
Also have you ever of a carburetor freezing in low 40F temps mowing leaves? I have.
I actually had one to freeze over at 70+F one time. Nearly crashed. If weren't for a carb heater I would have.
And JD will push their oil over anybodies other oil. Financial incentive. Personally I would drain both hydros and fill with the Tuff Torq 5w50 oil and no longer worry about operations.
Respectfully, we have differing opinions. I've run my equipment in temps lower than 25F and have never had a carb freeze up in 45 years, and that includes blowing snow. If there was a risk of carbs freezing in winter, why would manufacturers make snowblowers and blades for typical lawn tractors? I'm not calling bunk on your claims because different areas have different challenges, but 70+F and a carb freezing sounds like an aviation issue since temps dip with speed(windage) and altitude, and not a lawn mower issue to me. I've run my personal Z950M in lower temps than 25F, and haven't had an issue. Let it run for a few mins to warm the oil, and you're fine. Don't let yourself get hung up on the 'Deere pushes their own oil for money' thing. OEM's know what is best for their product because they've done extensive R&D on it in all temp ranges. This is how residential people mess up their machines by mixing alternative fluids from overthinking a simple solution, or Autozone pushes an additive onto them. Let's please not confuse them here by going into the weeds. The OP is fine by using the Hygard oil. Our farm tractors have been using that oil in sub-freezing temps for over 60 years in their hydraulic systems, which are more complicated than any hydro in a zero turn. The farmer knows to let the tractor run a little bit to warm the oil before working the machine. Let's please not overthink this and confuse the OP.
The Hygard will be fine. On a brisk day, under 32F, use common sense and just let it run for a few minutes before applying full throttle or load, and you'll be fine!