Oil for Ferret Fluid Coupling?

[LRDG]

At the Oshawa Museum, I found Dexron ATF in the fluid coupling of the Ferret I am working on. I notice significant creep/drag when in gear at a relatively low idle speed of around 400-500 rpm.

ATF has a viscosity grade of around 30 whereas the tables I have seen on various sites recommend a ISO 10-15 viscosity grade oil, based on the OM13 military spec. Seems logical the much more viscous oil we are using is the cause of the creep. What are you Ferret owners using?

EDIT: A second attempt at searching found a site that says, "DO NO USE ATF!" And to drain it, "you need to remove either engine or gearbox". Oh dear. Is there no other option? For example, where does the oil drain when the seal leaks?