Using the extended Thomas-Fermi plus Strutinsky integral approximation
to the Skyrme-Hartree-Fock method we show that provided the nuclear m
ass data are well fitted extrapolations to neutron-rich and superheavy
nuclei are quite insensitive to generalizations intended to simulate
the main effects of relativistic mean-field theory. In particular, as
far as energy-related quantities are concerned, it is sufficient to us
e the conventional spin-orbit force.