The topic of drain & fill of modern day hydro trannies has been discussed time and again in this forum. Using the forum search feature (found near bottom of tractor forum home page) will produce quite a few threads for your perusal. But, you need to be aware that you might need to put in "different key words" and do separate, multiple searches to find all the threads. Example, you might do a search using words like "hydrostatic oil change" or "hydro fluid change" or "hydrostat oil drain" or "hydrostat slipping" or "hydrostat overheating" or "hydrostat won't pull". Try to avoid using words like Toro Wheel Horse or any other words that are "too general" or you will be directed to every thread that has the word Toro or Wheel Horse in it. I'm not giving you a "brush off", it's simply that there is just too much info and too many variations for what you might have to do on your TWH to drain & fill the hydrostat. Do some searches, get familiarized with the search feature and read about what has been suggested in the past while you wait for a more specific response in this thread. You will probably also read about the correct way to operate your hydro and some other tips, do's & don'ts for hydros as well. Welcome to the tractor forum.

The key statement is that it won't go uphill. This also happened to my Wheelhorse. The drain on mine was a large hex nut, plastic, on the very bottom. It was very hard to remove because it was turned so far inward.

It's probably worn out internally. But, anyway, on mine, I kept changing the oil from 20W to 30W to 50W to 90W and I got a few years out of it and then it just wouldn't work anymore. Replacement Eaton transmission: $800 just for the part. Replacement was easy.

You can download the manual though Toro parts. You get to R&R the transaxle to change the oil. Keep the dirt out!

I have a wheel Horse lawn tractor that the hydrostat transmission slips when it gets hot I need a shop manual that shows me the rebuilding process