I've been very happy using Permatex Superlube teflon grease (obtainable at Radio Shack, where a few bucks will buy a multi-decade supply) on both classic front-element focusing threads and the helicoid in my Varexon 135 mm M42 lens. It's slippery, doesn't separate as some greases can, has no gritty residue as lithium grease often does, and gives a nice smooth damped feel.