We've got a ZF10 too, coupled to a Nanni 43. The original box died just after we bought her, with an unknown but fairly low number of hours on it. At that time I should have replaced with a ZF12 which is a drop-in replacement, but didn't know about it. Larger ZF's are NOT a drop in replacement.
I was a bit worried about longevity and did a it of research
. Found a site where there was much discussion of these problems, but no I longer have the URL. There was reasonable consensus that these trannies needed to have their oil
changed quite frequently - at each oil change
at 200 hour intervals -- and to be sure to use Dextron III rated ATF. You may have noted that the thing only holds 350mL of fluid, and I suspect this contributes to the problem. If yours is running hot (and I suspect that it is) you might consider their bolt-on external oil
cooler. Might make the difference and would be much cheaper than shifting to a different tranny.
So far we have about 2500 hours (eight years usage) and so far it is ok. We don't generally motor
hard, though, and the prop is correctly sized. I've got my fingers crossed -- for both of us!