Before a real expert comes along humour me with what I've done for refilling the Jatco box. I know you're supposed to drain the whole box and carefully run it for seconds through each gear to expel the old fluid. I decided some years ago that I would drain and replace with the approved fluid from
here till it reached the drain plug hole knowing full well that two or so litres would be left in. Run it for a few miles, up on the ramps, drain and fill as before thus diluting the mixture of old and new. If I was feeling wealthy I would do that again so that the original old fluid would be an insignificant proportion. It may not be strictly kosher but it's worked for me on my 150,000 mile Diesel Tourer. My own local garage - whom I trust - said they would not dare run the Jatco near dry and I got the impression that pretty well all garages would only drain and replace what they could more or less in the same manner.