Re: changing lower unit oil
The oil should come out of the lower unit looking pretty much the same as it did when put in. Clear and bright, maybe just a shade darker but not black.
Advisable to change every year, or at least to drain some off to check for water or milkiness, the first signs of water ingress, and top up again.
If I was you I would change it again after a short while. Two reasons. To make an early check for water ingress and secondly to ensure all the old mucky stuff is cleared out. I'd have probably flushed it with some cheap automotive engine oil.
Black ... don't like it.
