I use the same NLGI #2 grease that I use on the rack and pinion and linear bearings. I take the excess grease from the linear bearings and wipe it on the ballscrew with a gloved hand (Nitrile or vinyl). That seems to work just fine. After it begins to pile up at the top and bottom I take a Starbucks stir stick (or any small piece of wood) and redistribute it on the ballscrew.
David