I start my HST tractor and immediately put it to use at temperatures as low as about 10 F. Just don't rev it to "PTO" speed right away, but it is started up with the throttle set to get about 1,200-1,500 rpm. I usually keep it below about 2,000 rpm until it gets fully warmed up.
The local JD dealer recommends use of the low vis hydro fluid here in Virginia. If you used the higher vis stuff, you probably need to let it warm up some more.
If you run an HST in temperatures much below about 0 to 10 F, I'd be in favor of getting a warmer for the HST. Just a small heater placed underneath the HST oil reservoir area for an hour or so before startup ought to be enough. Be probably more economical and safer (for the engine) than letting it sit and idle for a long period.
Ralph