Saildrive Fluid
My boat, a 2005 Bavaria 36, came with ATF (automatic transmission fluid) in it, and I have used this since without any problem. You can also use the same oil recommended for the engine, a 15W40 oil. Look at what is in there now, it is easy to tell the difference (ATF is red). I would continue to use what has been used in the past if I were you. If I remember correctly, the fluid should be changed once per year or every 200 hours, whichever comes first. I do mine at the annual haulout when I can remove the drain plug on the bottom of the drive.
Good luck,
Tom