  We study the $3d$ substitution in $4d$ transition metal oxides in the cases
of $3d^3$ doping at either $3d^2$ or $4d^4$ sites which realize orbital
dilution. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mott
insulator with different ionic valencies, underlining the emerging structure of
the spin-orbital coupling between the impurity and the host sites and
demonstrate that it is qualitatively different from that encountered in the
host itself. This derivation shows that the interaction between the host and
the impurity depends in a crucial way on the type of doubly occupied $t_{2g}$
orbital. One finds that in some cases, due to the quench of the orbital degree
of freedom at the $3d$ impurity, the spin and orbital order within the host is
drastically modified by doping. The impurity acts either as a spin defect
accompanied by an orbital vacancy in the spin-orbital structure when the
host-impurity coupling is weak, or it favors doubly occupied active orbitals
(orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic or
ferromagnetic spin coupling. This competition between different magnetic
couplings leads to quite different ground states. We find that magnetic
frustration and spin degeneracy can be lifted by the quantum orbital flips of
the host but they are robust in special regions of the incommensurate phase
diagram. The spin-orbit coupling can lead to anisotropic spin and orbital
patterns along the symmetry directions and cause a radical modification of the
order imposed by the spin-orbital superexchange. Our findings are expected to
be of importance for future theoretical understanding of experimental results
for doped $4d$ transition metal oxides doped with $3d^3$ ions. We suggest how
the local or global changes of the spin-orbital order induced by such
impurities could be detected experimentally.
