A few years ago when I had a boat with an 800 I used the Merc high performance gear lube, worked great, kind of pricey though. As for capacitys download a manual from the adults only section.
What year is it? If it's an electric shift, there are special requirements. If it's a mechanical or hydro-mechanical, it uses 80W outdrive gear lube in both upper & lower. Like this:
