Firstly: Fill the gearbox to the level described by Christian (or Beccy, not sure...)
Next: Go and have a cup of tea (seriously), then, (after ½ an hour) top up the level again.
Finally: After your first road test, re-check the level again (cold).

You should use a full synthetic 75W/90 gear oil of good reputation (not some shit cheapo gloop that you got down Cost-Cutters)

There is no real running in required with our boxes, other than, I would like to think that you wouldn't rag your engine when cold. Well, provided you've brought your engine to running temperature, your gearbox will have a good circulation of oil about it, and that will be good enough!
Using a full synthetic oil will mean that you shouldn't ever need to change it (there is a nice big magnet in there to catch all the bits of reverse idler you knock off with your over-enthusiastic engagements of reverse gear!!)