I'm going to shove something out a this juncture, if one had a dead 
viscous coupling, perhaps if indeed the stock fluid is unavailable, 
and likely not sufficient anyway, as it seems the phase change may 
have been related to the pressurization imparted before it was 
sealed, after all pressure often effects phase change doesn't it 
(thermo is way back I'm a solid mechanics guy), and likely your seals 
are shot which bled the pressure anyway (a good reason never to 
unhook the front as seals dry when not used (things not spinning). 
MAYBE it would be better than nothing to fill a dead one with a 
highly viscous silicone of viscosity near the thickened VW fluid 
stage.  It would gain you an admittedly arcane system, essentially 
center diff locked so 50/50 torque split at all time, far better than 
nothing.  And it should not have any extreme effect on handling or 
mileage,  the old Q syncro once broke with the center locked and I 
was some time gets the parts from Germany to fix, I saw no real 
mileage detraction, and you still have full left to right 
differentiation.  Seems a better thing to be looking for as a last 
resort measure IMHO instead of the exotic and likely insufficient in 
itself VW fluid.  I'd bet that for offroading this would be in fact a 
superior solution.  Hell has anyone considered modifying for a 
mechanical front/rear diff like the quattro system in fact, a lot 
more reliable than the hokey exotic viscous stuff.  One more final 
bit of interesting conjecture.  They make a viscous coupler just like 
the syncro to retrofit to full size chevy blazer, I saw the ads 
myslef a few years back but remember no specifics, worked EXACTLY 
like the VW solution so somebody else must make this stuff.

John
vwbus@netbiz.net