I am not knocking anybody's suggestions, I am just going by what I have experienced myself.
I have opened several trannys that have more than likely been closed up since their birth 50+ years ago. I am sure most of the previous owners did not change the fluid in the transmission if they were having no issues whatsoever. Remember the phrase: "If it aint' broke, don't fix it."
The fluid inside these transmissions were dark, and somewhat thicker from years of use. There was little to no evidence of moisture, proving the Hytran absorbs moisture very well.
If the Hytran back in those days lasts 50+ years, then imagine how long today's Hytran would last!
30 wt. oil is ok, but it is thicker, and the temperature range on 30 wt. oil is less than the Hytran.
My preference:
If you have the tranny out of the tractor, I would take the rear cover off, along with the axle housings, and clean everything along with replacing the axle seals. That way you can inspect for any damage, or debris that may be lying in the bottom of the tranny. If there is no debris or layers of "metal dust" lying in the bottom, you know your tranny is in pretty good shape.
Why change the axle seals?
I would hate to change just the fluid, only to find out the axle seals were leaking. Remember, the seals have been in there as long as the fluid has!
CCSpecialties has the seals for less than 8 bucks each.
If the tranny is still in the tractor, and you do not want to take it out, you could still change the axle seals.
If you put any type of fluid in the transmission to clean out the old fluid, you will have some of that fluid remain in the tranny and axle housings.
Taking off the rear cover and wiping out the tranny will get most of the old fluid out, except for what is lying inside the housings.
Raising the front of the tractor will help the old fluid to run out of the tranny also.
With a little effort, you can clean out the tranny, replace seals and gaskets, and fill it back up with Hytran for probably less than 75 bucks.
Then you will have a tranny that should last another 50+years!