Old, heavy grease can cake up the ballnut enough that the self levelling stuff can't get in there. IMO disassembling the ballnut and cleaning it and the balls/guides completely are vital to ensure smooth operation. If there's wear present it's most likely gonna be the balls/guides/nut as the gears themselves are pretty durable and have lash adjustment available. I used a cake decorating bag and a spatula to fill mine.
