I'm pretty sure I used 75w90 Synthetic gear oil from Fruchs (originally Silkolene). - It may depend a little on how hard you drive, and how often you intend to drive it, but I just bought the stuff that said "suitable for fill-for-life applications".
Not ATF fluid if that's what you've been told; that may be suitable for the internals of the viscous coupling, but as it's a sealed unit you shouldn't be touching it.
I'm sure Danny will correct me shortly if I've got this completely wrong.