:welcome: to the forum farago!
I own a MF50F. It is a 1984 Massey Ferguson back hoe(industrial machine, painted yellow). It has a two speed trany. High and low transfer case. So it gives you a total of four speed range. It has a torque converter, and is controlled via two side by side pedals, activated by your right foot. One pedal is for forward motion, while the next one will be the reverse.
The transmission has a set of clutch plates which are activated by hydraulic pressure, provided by the converter. The more you press on the pedal, the higher the engine RPMs. Therefore, pressure increases proportionately to engine RPMs.
I do not know about your machine, but with the description of mine, I hope it helps. So the answer is(for my machine):Universal Tractor Fluid.
It is very close to Hydraulic fluid. You can buy this UTF at any Wall Mart Auto section(it is the cheapest I know of). You will notice this same fluid is being used for the rear transaxle, as it is directly connected to the transfer case, which is directly connected to the transmission. All three components share the same oil. This whole system contains about 60 liters.:tractor: