80w90 mate. good time to do it is the summer as the stuff is really like toffee in the winter... (or more appropriately, lithium grease

)
allegedly - in the other gearbox post (or three) it's worth your while seeing if you can pour it in thru the speedo cable hole. open the top "filling" bolt and fill thru the cable hole til it oozes out the fill hole.
(as ever, make sure the top bolt can be undone and replaced before touching the lower one!)