Automatic transmission fluid; a tiny amount brushed on with an old toothbrush every week or so. I ride almost every day, so this works out to be every 500-ish miles. I often forget, so sometimes go longer. The ATF is sorta self cleaning and doesn’t leave sticky, gummed up crap all over the swing arm like some of the spray lubes. Any ATF that flings off wipes off quickly and easily with a paper towel or shop rag.
Edit: I keep meaning to switch to one of these
to make life even easier on myself.