technically nothing, buts its nice to have something on there so things hopefully wont rust or fuse together etc... I have been using more than anything as of lately something from NAPA called Sil-Glide its a silicone based grease thats for brake parts. not supposed to freeze or melt and its not as messy or at least cleans off my hands and parts easier than bearing grease does. I just thin fingercoat axles and stuff like that during assembly.
