It isn't just the viscosity, the additive package of "universal tractor fluid" and "hydraulic oil" can be very different. More anti-wear additives, plus friction modifiers for any clutches/brakes that are in the compartment.
The hydro system on those tractors tends to be "whiny", I doubt you will see a big difference after the oil change.
If you want to put a premium product in, I have used this in the past. It pours at -50, great for cold weather: http://texasrefinery.com/images/lb-speciallowtempuniversaltorque(1).pdf
I can get you a price on a 6 gal. pail if you want.
Also, your local oil "jobber" probably has a Shell/Mobil/Citgo product that meets the J20D spec, give them a call.
Edit: Waldo beat me to it.