Not the answer you are looking for but food for thought. I noticed that JD 303 vs J20c seemed to be of a higher viscosity. I like the 303 for that reason but is questionable in wet brake and hydro steering applications for which the 20c is blended.
In looking around and reading about pumps and oils and all, I found that you have to be careful with hydraulic pumps and the types of oil being used. Seems that under extreme conditions, like hard winters for example, the oil can get too thick and the pump goes into a starvation mode and self destructs. Don't take my word for it, www the subject yourself if interested and read about hydraulic pump, types and physics and no-nos.
I have a new small tractor that is all old school mechanical except for hydro steering. I'd love to use a gear oil to make shifting quieter and smoother but don't feel that I can't due to the pump and the steering requirement. Not worth the convenience to experiment with the oil type.