I know lots of people here have used the tractor supply Traveller Universal Tractor Trans/Hydraulic Fluid. When I changed my oil 3 years ago I used it and haven't had any issues. It's supposedly compatible with hy-tran, but I wasn't mixing old with new either. It's not really transmission fluid that you need, it's a hydraulic fluid since you don't really have a transmission, you have a hydraulic pump.
https://www.tractorsupply.com/tsc/pr...l?cm_vc=-10005