>>In order not to destroy this very expensive viscous coupling, could anyone
>>tell me if it's OK to mount chains on the rear wheels or if you have to put
>>them on the 4 wheels?
>Bentley recommends that chains be used on all 4 wheels of syncros.

Since you're already in wheelspinning situation, minimal traction etc. the
windup of the viscous coupling won't happen.  Too much other squirming going
on already.  But wearing chains in continous 'easy' driving, ie.10" of snow
on the roads at 30mph, won't likley be as forgiving on the coupling.  Really
worried?  Air down the chained up wheels until the height of the axle centre
is the same as it was before you mounted the chains, will keep the rolling
diam. same that way.  How often do you check your tire pressures in summer?
;)  I use chains on all 4 wheels when in muck/snow, but only for the
shortest amount of time.  Just don't like them.  Tim