A bit of history of my GC2300 . Hydro filter ruptured on a cold winter startup. 35 hrs on the factory fill hydro oil. 50 hour service done early by dealer with a "dealer recommended" oil. IIRC at 120 hours ( 2yrs later) the fluid was so contaminated with water that valves started freezing. Looked almost like chocolate milk. Changed it out myself with Petro-Canada Duratran Synthetic. Just did the 500 hour change this fall, and the oil still looked really good after 8 years in the GC. I believe the Kubota Premium UDT was a repackaged Duratran Semi-Synthetic. It may still be. That is why I recommended it. Front axle uses the same fluid. Full synthetics cost has been negligible when compared to the longevity of the fluid, and the easier operation in extreme temperatures.
My tractor lived under tarp for a few years, this may have contributed to the water contamination. Keep the tarp tight, or place a softener underneath, so if a strong wind starts the tarp flapping , it doesn't rub off the paint.
Only place a hydro heater so that it cannot be damaged. If you don't have guarding to protect it, it will not last long. Running synthetics with a battery tender and block heater should be all you need.
just some guy on the internet.