There actually is a speedo cable lube that is sold in auto parts stores. I believe the active ingredient is graphite.

But speedo needle flicker is usually caused by a kinked cable. Make sure the cable travels through the various areas unimpeded and there are no sharp curves (or kinks).

It's a bit early in the car's life to warrant cable replacement unless it was damaged in some fashion...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car