Better still, call ZF Marine (who bought Hurth
who built your transmission) and ask them what to use in the tranny. My guess is they'll say any good ATF is fine. They will also say, as will almost any conscionable tranny shop, do not, not
use synthetic ATF. Yes, it will keep the gears happy. But it will also attack little brass parts in the clutch
pack and that will kill the tranny deader than Dickens.
ZF's Stevensville, MD office number: 410-604-3320
DISCLAIMER: I have no connection with ZF Marine, save as a customer.