My horse crack, I mean my shell cordovan wallet would thoroughly enjoy being rubbed with Venetian leather balm. I find a swirling movement with the finger works best. By localising the pressure, your finger will increase the surface temperature of the crack, allowing the balm to penetrate deep into the fibres. Best to do this to the beat "I touch myself" by Divinyls, With your finger throbbing to the rhythm, the surface temperature and friction achieves equilibrium for optimum effect.