Precise resistivity and viscosity measurements for ionic mixed melts, regular and glass-forming, have been performed on the liquid state over a wide range of viscosity variation. Their universal behaviour in the high-temperature region allows us to scale them relative to their activation energy. The resistivity-viscosity relation reveals a fractional-power character over the whole range of parameters investigated. This interdependence suggests a fractional character of the Stokes-Einstein law of the viscosity-diffusion relation for ions in melts.