I see you have a Sunstrand brand transmission, hopefully you can get it serviced. Since the mower is about 25 years old it is probably due for a transmission rebuild. The tolerance on the internal parts needs to be very close to work and with age and use the internal parts have worn to the point that the fluid is bypassing when hot and thin (lower viscosity). Hope it is a rebuildable component for your tractor, keep us posted on what you find out.
