What do you use to lubricate a noisy speedometer cable?

Never had any concerns using a high quality light engine oil, ATF or good additive such as Duralube. Cable removes from top so oil can be squirted in or gently applied as shoved back in. Greases can tend to be too thick or gummy and cause irregular rotation inside casing - which can, as a result, lead to a jumping needle. Do check how assembly is routed from top to bottom so no sharp curves. Jumping of speedo needle can also be due to stickiness or wear in that upper mechanism, but such issues in the speedo head require a lot more effort (or expense) to correct. So use what you wish, for casing and cable, and hopefully problem solved. Remember also that the cable rotates the same as the nylon coated drive gear and, graphite is a 'solid' so I would no more use it in the casing than I would use it in my door locks.