I believe the Amel employee told you correctly to use grease, not oil (even 320 viscosity). It is basically a winch, not a high speed gear box and you should grease it like a winch. The oil will leak out, although, like bleeding, it will stop eventually :-)
I don't know the source of the recommendation you note to use 320 oil, although Kent and I discussed that on this forum as being the original oil in the Bonfiglioli (or other brand) "reductori" reduction gear boxes that are driven by the electric furling motors. Many of us have changed that out to conventional grease with Zerk fittings added for periodic replenishment, although in that appllication either should be fine.
Hope this helps,
Craig - SN68 Sangaris, Tropic Isle Harbor, FL