Thete have been threads over the years asking if you can modify the Sierra viscous LSD by changing fluid viscosity
I searched for how these diffs work and if modifying was possible.
Here is what I found....

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.

Further information I have found...

It seems that the LSD part of the diff is sealed for life, you would need to drill and tap to change the fluid.

Some diff builders purport to be able to swap out the fluid to change the diff effect.

Ford has (somewhere) a torque figure to use to test if the fluid has degraded, jack one wheel, put torque wrench on hub nut, measure torque required to turn wheel.

not cheap, but this lot offer a rebuild/retune service for them: http://www.gearboxman.co.uk/psdiffsgearboxman.html
(although I've never used them so I can't vouch for there work)

[Edited on 17/9/2012 by mcerd1]