EDIT: sorry missed the "low viscosity" in the subject line...
There are a number of universal tractor trans hydraulic fluids on the market sold by Tractor supply Napa etc that claim to be equivalent. I have used these in my geared trans JD without issue.
A hydrostatic trans is a different beast altogether with tighter tolerances and much more heat generation demanding a lot more of the fluid. Although many do it without issue, personally I would hesitate to trust such a high dollar component with the generic fluid.
On the other hand the argument has been made that you just spent many thousands of dollars investing in a tractor so you shouldn't balk at spending a few extra bucks to get the manufacturer engineered fluids to protect that investment.