The newer model 2000 and 3000 Cubs look to have synthetic hydrostatic fluid put in them from the factory. I don't know what year this was first used. I would first find out what the manual recommends for your hydro.
Either way, synthetic or not, I would use only a fluid made for use in a hydrostatic tractor transaxle. The regular (non-synthetic) Cub Cadet branded fluid is about $3.50 for a quart bottle, not too bad considering you would be taking no risks, regular ATF supposedly does not have the temperature stability charecteristics or detergents included in the OEM hydro fluid. They synthetic from CC is near $9.00 a bottle, but in the case of the hydro has a lot longer interval between changes.
As hydrostatic transaxles can be expensive units to repair, I would take no risks and use the good stuff with a fluid change.
-Fordlords-