I have just had a quick look on YouTube and I found a video how to service the viscous unit viscous unit unfortunately it is in Russian.
It is fairly self explanatory. The guy changed the oil and freed off the clutch. Bell engineering must do the same.
Apparently it is a silicon oil @ 10,000 cp viscosity and takes 250mls. Don't throw your old one away just yet.
The viscous unit is supposed to slip at slow speed and act like a central diff does. The principle is that the oil viscosity increases at high slipping speeds, like when a wheel is spinning, and it grips more. The oil thickens up.
When I have tested mine on the car using a bar on a wheel hub, there is high resistance even at low speed. It needed a lot of force to turn it.
Interesting comment down below in the video comments section that Ashcrofts disagreed with his method.