I believe they should be greased, based on all the comments I've read here. To stop the oil leak you could replace the O-Ring on the black collar of the winch socket, but of course most of the oil will drain out when you remove it.
The top bearing in this manual gearbox will not be lubricated by oil at the bottom of the gearbox, and it is especially subject to corrosion with water running down from the mainsail right on top of it. The oil seal should keep the water out if it's in good condition, but they have a lifespan and at the end they leak.
I would remove the gearbox, disassemble it, and be sure you had grease on the top bearing, and that it's in good condition. Many people here are adding grease zerts to the gearboxes driven by motors so they can add grease annually. I packed it with a good quality wheel bearing grease. Take special care to apply a liberal amount of grease to the top bearing and fill the top oil seal with grease.