  Spin-orbit splitting is an essential ingredient for our understanding of the
shell structure in nuclei. One of the most important advantages of relativistic
mean-field (RMF) models in nuclear physics is the fact that the large
spin-orbit (SO) potential emerges automatically from the inclusion of
Lorentz-scalar and -vector potentials in the Dirac equation. It is therefore of
great importance to compare the results of such models with experimental data.
We investigate the size of $2p$ and $1f$ splittings for the isotone chain
$^{40}$Ca, $^{38}$Ar, $^{36}$S, and $^{34}$Si in the framework of various
relativistic and nonrelativistic density functionals. They are compared with
the results of nonrelativistic models and with recent experimental data.
