I found this thread in the nick of time! I recently got a nice Vitesse with a j overdrive, and had exactly the same symptoms Freyasgrandad described. I had assumed it was something awry with the pressure relief valve, or an electrical fault.
I took the trans cover off yesterday, to see if I could find anything obviously wrong. Wiring fine, so I topped up with Ep90 while I was down there. The gearbox took 1.2 litres before it was full! (I think the capacity is around 1.3l?).
Overdrive fine now, and no leaks so far. A lucky escape ????