The linear rail manufacturer, Hiwin, recommend a NLGI #1 grease without high pressure additives. The ball screw is a no-name Chinese off eBay and the seller said grease was fine, so who really knows. I used Mystik JT-6 #1 grease, it's what Onsrud recommended for the router we use at work. If its good enough for that $350K router it should work for me. If the ball screw fails I'm not out all that much and I'll buy a better screw to replace this one.