I find it interesting that two substantially different fluid compounds [i.e. motor oil and transmission fluid] run neck n' neck in experienced boater's opinions for working best in BW VD marine transmissions.
From what I understand the most recommended fluid for BW VD trany [back in the day] was a Ford trany fluid, I forget the number affixed to it.
Talking with my decades trusted transmission rebuilder as well as some knowledgeable marine mechanics and from learning [readings] in forums I have stuck with Dextron III. Sure stays clean looking and our two BW VD tranys work well.
I just don't understand how very slippery [and I believe somewhat compressible, lubrication prone] motor oil in contrast to not so slippery [and I believe not at all compressible, hydraulic prone] trany fluid can each perform as well as so many say in the same transmissions.
Am I missing something here??
Cheers!