You ideally want something a bit thinner than the more common greases.

Try searching for fluid or semi-fluid greases, which are designed for autolube/centralized lubrication systems. I just done a quick search, but could only find large packs.
Although this just came up - http://store.forestlubricants.co.uk/...-fluid-grease/
Something like that, provided there is some form of sealing to keep it in the gearbox. It's got a consistency a bit like yoghurt, in that it's thick but still pours.