The info I have is that it uses 10w-30 reg engine oil.
I just tore my hst apart today is was slipping so bad I could not go forward or back I removed the entire rear end and hst from the tractor and opened up the hst. The oil was black, thin and full of metal flakes. I could not see any significant damage to the unit other than some wear on the spring that puts back pressure on the driveshaft pump. The internal filter was plugged also. I flushed the entire unit and refilled it with 80/90 gear oil to prelube. I will tip the unit upside down to drain all the heavy oil and refill with the lighter stuff. Hope this all works. The hst itself is a pretty simple setup, two spring loaded rotary pumps and a drive/shift slide. I think the real problem is that there was so much metal flake in the oil that it actually jammed in the small bypass valve that alows manual moving of the machine. The valve was stuck in the free position...no wonder it wouldn't move in forward or reverse. Is there somewhere to access a parts diagram for the hst?