Speaking in terms of actual RPM's from one prop to the other is not acurate IMO. You are talking about you specific engine a lot varys from one engine to the other and not just fuel type the diffence in 500 rpms higher or lower to be should be considered a buffer. In all practical circumstances it is quite safe to say that the 13X4W and 13x6 have the same loads imprressed upon them on a specific make and size engine. Just MO