Its best left to the professionals. The below is copied from a Ford XR4x4 site.
The viscous differential also has interleaved plates, and in fact works because of the viscosity (internal friction) of the silicone fluid that fills the gaps between the plates. The plates therefore do not actually touch one another. The percentage of torque transfer varies with the difference in speed between the two wheels, partly as a result of the plate design and spacing, and partly because of a physical property of silicone fluid that it experiences virtually no change in viscosity when heated. If one wheel is rotating only a little faster than the other, only a little torque transfer occurs, when there is a big difference between the speeds of the two wheels, the viscous differential will transfer up to 95 percent of the torque to the wheel with traction. Because of the difficulties in handling the silicone fluid, servicing a viscous differential can be accomplished only with special equipment. The silicone fluid is under pressure, and there is an air bubble of known (and controlled) volume included in the differential casing. Any deviation from the correct amount of pressure or air bubble volume can radically change the characteristics of the differential.
Not sure of the relevance to Scoobies but I imagine there much of a muchness.
Last edited by Rescue Dude; 25 February 2011 at 03:24 PM.