Some years ago I studied this topic a little bit and came to a conclusion that I will use 60000 cSt silicone oils in my Sierra’s center viscous coupling unit. Today I did some Googling and tried to verify do I still agree with the outcome. It seems to be as hard as earlier to find proper facts about this. So I will use the stuff I ordered some years ago:

This fluid is meant to small RC cars, but based some real life tests by the others it seems to do it’s job quite well. I expect that I need to consume 2,5 bottles until the viscous coupling unit is full, but we will see that later.  There are different viscosity fluids available for RC cars ranging from some hundred cSt to 500000 cSt. By what some UK forums are saying you should not use more than max 300000 cSt fluids and there are some bad examples what happens with too thick syrups.

By current knowledge I have I’d say 50000-100000 cSt is the optimal range for normal use car, but people seem to have achieved some good results also with 12500 cSt Scania viscous fan fluids (561072). So beats me what should be used since this seem to be a secret science, but my choice has been made and I’ll use 60000 cSt. Unfortunately it most probably will take quite long time until I’m actually testing how this is working since this is my spare viscous coupling unit I’m working with. And in my car I have a brand new (bought some years ago) Sierra Cosworth viscous coupling unit installed at the beginning of 2013.

Wikipedia article about viscosity:
http://en.wikipedia.org/wiki/Viscosity

I also managed to find some specs about the sealings and seems that they should be X shaped O-rings XR 5.33×47.00mm. People seem to have also used standard O-rings with good results, but I think having original like sealing with double contacting surfaces should be much better. But let’s see when I visit local bearing shop with the parts and old rings since they usually are able to give really good hints.

http://en.wikipedia.org/wiki/O-ring#Other_seals