Iím amazed! Five pages of comments about oil. My Twin Disc says change oil at 1000 hours running time. There is no combustion so no worries about that type of contamination. With the average of 200 hours a year, many on here may not even own their boats for 1000 hours. Which is a digression I admit. ZF? Borg Warner? Really?
Manual or manufacturer, the rest is just opinion, rumour and guessing.
Don't believe everything that you think.