Mike,
You may have to grind the fins off the mainsail furling gearbox to fit in the mast profile. So you’d need to paint over anyway.
I changed mine three years ago. I painted with an aluminium primer and top coated with a one pot paint that matched the mast/boom. It’s worked fine, and if it gets scruffy I can paint it again.
I would say that one of the gearboxes was filled with oil, the other with grease (there was a discussion on this) The oil dripped out (in small amounts) of the seal onto the deck, so I refilled it with grease instead. Don’t know when you last removed the boom gearbox, but I take mine off every two years now to keep the outhaul shaft easy to free.

Ian

Ocean Hobo SN96
On passage to North Wales