"Please note that all viscous clutches MUST be stored upright – i.e. in the same position they are in when installed on the engine. This is to ensure the oil does not leak out, because if it is stored flat (on its front or back) it will leak and will fail."
http://www.masterparts.com/products/...g/fan-clutches
Also, I have heard that the silicone fluid can leak and contaminate the bearing lube.