I’m just going start dumping links here that relate in some useful way to understanding how to test the function of a viscous coupling such as installed in the Vanagon Syncro.
syncro.org page. Not sure about conclusion derived from graph of viscosity vs. heat though, but still good info here.
Volvo vc test, in car, interesting – FHPs_SB46-0005-0101
Natterings on the North American Subaru Impreza Owners Club, extracts from a SAE paper on how the VC works (not bad, still does not fully explain how the plates are forced together in the “humping” stage, its still a mystery). This Subaru site has a few discussions about vc function and testing, its worth a look around.
“Out of car” testing of a Subaru vc. Autospeed article.
Test for Landrover vc described here at transmission specialist. Look under “technical information”