Another update for Syncro owners, some pay dirt on the Dow-Corning avenue.
Now have a copy of the VC design paper, SAE Technical Paper Series # 860386
'A Viscous Coupling in the Drive Train of an All-Wheel-Drive Vehicle' by
Wolfgang Peschke, VAG.  This is the full details of developing the Syncro
coupling, fluid is a siloxane goo.  Lockup occurs due to PRESSURE increase
inside the coupling caused by the temperature rise 5-10% air is inside
coupling when filled, and pressure may go to 100bar (1500 psi) when locked
up. I'd gamble that some of the VCs die simply by seal failure and leakage
at these pressures.  He also mentions that serious sustained overheats will
permanently up the viscosity, leading to fulltime lock up, and I think I've
read of one such failure.

When I've gone through the paper and another SAE paper #920611, 'VC in 4WD
Vehicles: Application of Computer Modelling'  by some people at Chryslers
'New Venture Gear' company which models the SYncro VC exactly I'll post
results.  Will xerox copies as desired, but wait a bit please.  Also have a
paper of unknown origen discussing viscous differentials, very slick.  Like
I said, pay dirt from the D-C guy.  Tim