I've done some more research and have found the ZF Hurth repair manual http://www.bukh-bremen.de/files/346/...s%20ZF85IV.pdf
but now I'm confused. The Mastercraft manual says to use 15/40 oil but the ZF Repair Manual says to use Automatic Transmission Fluid. Which ones right I've got 15-40 in at the moment?
Having read the manual the gear selector definitely works under hydraulic pressure so I guess this is my problem (it runs around 300 PSI). So perhaps I've got the wrong oil in it?