I think one of the biggest problems with the oil in our old tractors is moisture.
bad gear shift rubber boots is a major moisture source.
but also, the gear case can sweat and in time you will get some moisture buildup.
my buddy would open the drain plug just a little and get a small amount of water out, if the tractor had been setting for a long period of time.
I always have used the GL-1 trans/hyd oil in my 1957 TO-35 and so far it's worked well.
some say to warm tractor up to normal operating temp, then drain trans/hyd drain oil then add like 5 gals of kerosene to system and run it a bit then drain again
I do not know about this: I bought a fine running old ford car motor once, but before I pulled it and installed it in my other car, I ran kerosene through it for a few minutes.
from then on it smoked like an old freight train, I ruined that nice running 292 ford motor in about 5 mins.
so, no kerosene in the trans/hyd sump of my little tractor.
I buy the GL-1 oil at NAPA, best price I have found, they can order it for you, if none is on the shelf.
My understanding: something about using GL-1 oil because it properly protects the yellow metal in the bearings of the transmission, am not sure of that, but if it calls for GL-1 then that is what I am going to use as long as I can get it.
you are to use GL-1 to protect the transmission bearings, not the hyd system???????