From memory,12 oz. or so. I have been using Polaris synthetic chaincase oil but there are many threads in this forum with alternatives. Some use ATF, Amzoil, Klotz, etc. I would suggest that proper oil level and change it regularily is key. I do it every fall for sure but I don't put the miles on that some do. Of course I also change it whenever I'm in the chaincase for whatever reason. Second suggestion, don't drain with the plug and refill. It's worth the effort to take the cover off, which is pretty easy, so you can get a good visual on the chain and gears for wear, obvious broken links, etc.
Cheers,
Gary