Interesting. I am surprised Yanmar would have such a wide range of lubricants. I know of older BMW cars that when the manual transmissions are hard to shift (especially in below freezing temps) some have used ATF in the gear box instead of the recommended gear oil- and it seems to have worked.
I have seen many transmission or gearcases spec different oil and or lubes depending on whats available
for example many recomend atf now especially the dexron 4 stuff which is really good
here is an interesting tidbit too
many motorcycle suspension shops and or mechanics prefer to use dexron 4 atf for their suspensions as the detergents and antifriction properties as well as antifoaming agents are preffered over the overpriced "suspension" fluid
its also widely known that many fork fluid is simply rebranded atf...but wuth 3 or 4 times a higher price
x2 on your bmw comment...
in any case...the kanzaki trannies are pretty robust and im sure any old bacon grease can work for a while
if it were me I would use what it had in there before me or if you want to change I would tear it apart clean meticulously and use your preffered lube, be it oil or atf...having said that, some clutches and or plates or disks will retain a fair amount of the old lube in them even after cleaning so its at your own risk when doing that
for example in wet clutch engines or motorcycles plenty of folks get all sorts of mad when simply switching brands of oil...or when they switch to an energy conserving oil...and creat all sorts of havoc...
in some cases its the oil...others its the owners not rinsing or sufficiently eliminating the old traces of oil from the clutch