From my feeble research this is how a fan clutch works;
The part that the red arrow is pointing to is the hub, bolted to and driven by the water pump pulley. The ribbed area is where the silicone oil shears, transferring drive torque to the housing that the fan is bolted to.
The blue arrow points to teeth that work as an oil pump.
This picture is of the front housing. The blue arrow is where the "oil pump" pushes the oil into the reservoir. On some clutch designs the oil leaks into the active part of the clutch when it's stopped and has to be pumped back into the reservoir when the motor is started, causing the noise on startup.
When the motor is running most or all of the oil is in the reservoir until the thermostat opens the valve (red arrow) allowing the oil to circulate and shear transferring torque to the fan. When the thermostat closes the valve the oil is pumped back into the reservoir allowing the clutch to freewheel.
The blue arrow points to the area where the oil enters the reservoir. The reservoir capacity is the open area in this picture minus the red area. So if one was cleaned out and the reservoir filled with water, the water measured, subtract the volume of the red area (what 1.5" x .25"?) we would know the correct fill? I'm betting ~3-4 tubes? The new clutches I have had apart had about this much fluid in them, nowhere near full. If the whole clutch is filled, the oil would always stay in the clutch area and not freewheel?
They are not a hydraulic coupling, their silicone couplings, no other type of oil that I have tried works, at all, not even a little bit! They work because of the unique properties of the silicone oil. A bunch that I have taken apart have appeared to have the proper fill amount and didn't work well. When cleaned and refilled they worked as well or if thicker oil was used better than new. Leading me to believe that the unique (magic) properties of the silicone goes away, maybe ruined by heat, age, who knows?
Like I typed this is what I have been able to find in my feeble research and experience from refilling a bunch of them. So if anyone has better info please add.