Having worked in the trailer repair business for a number of years, You should never grease your trailer ball. It attracts sand and grit and wears out the coupler fast. I don't know how many I have replaced. Also after a lot of hard towing the trailer ball should be replaced, have had some break off at the stem due to metal fatigue. I no longer tow as much anymore, but at one time had a trailer hook up to my truck almost 24/7.