I just changed my hydraulic fluid again this year. It was only a couple seasons old, but the original oil when I bought it was a mess so I figure it will help to clean it out to change it often for a couple years.
Anyway, drain everything well. I took the PTO out and reached up in there and scraped as much gunk as I could into the drain hole. I had the top cover off the lift and scraped all the gunk out. The transmission came last, and it too had gunk and water in it. A real mess but I used some rags to get everything out as much as possible. I even used some kerosene to wash some crap out and then wiped it out and let it sit to dry out.
Instead of heavy oil, I used some hydraulic/transmission fluid in it and it has worked just fine. I just filled it to the weep holes and pluged it back up. That fluid works good for me winter and summer.
Itís a messy job, but youíll be making a tractor real happy doing this service.