find out what mfgr (hydrogear, tufftorque, etc.) built tranny for that unit. From that, s/b able to find fluid type (often 20w-50-w motor oil) is specified.
Will prolly hafta pull tranny and turn upside down to drain.
Take plenty of pixs during process to make it easier to re-asm - keep everything as clean as possible (hydro trannies DON'T like dirt internally) - Catch/measure all old oil so as to know how much new oil to add. Be patient/take your time - drain/refill process will be s-l-o-w.
Might want to verify drive belt tension good and tranny cooling fan undamaged/ tranny case clean before starting R&R - heat is the tranny's enemy.

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hey there, the problem with your transaxle/transmition is not the fluid. the factory flaw is in the "center case" which mates up to the pump and motor. well on the faces of the center case you will see two crescent shaped openings. they are oil galleys and one side is high pressure and the other low and as the pump/motor spins, the high pressured oil slowly erodes a channel between the two port creating a significant pressure drop/leak in the closed hydraulic system which equals lack of power!
Tuff torq is the manufacturer and they know about the design flaw and sells a rebuild kit for $376.64 but instead of falling for their fix it kit, you can go to their website and pull up all the information needed to take it apart, mill the surfaces or have a machine shop do it for $20 and then put it all back together yourself in an afternoon. tufftorq recommends putting 5w50 synthetic oil back in for reduced wear. they also sell filters for $10 which I would recommend along with drilling and tapping drain plug holes for the two chambers inside. on their site, you will find the directions for this and you can buy the plugs from them or just go to your local true value and get them. another little thing you should do is remove the glaze from the drive pulley with some medium grit sandpaper. just roughen it up a bit taking care to remove any rust, burrs or sharp edges so the belt has something to grip to. and speaking of belts, I recently broke the drive belt for my snow blower attachment and found out that most places wanted over $50 for that belt. JD was 70+.!. well, I found a company that sells that same belt for $9.!. its vbeltsupply.com and they FedEx it too!
i'll shut up now, but I hope I shed some light on your problem
anyway, its not that difficult to do yourself.

image of one with no wear