  The sub-barrier capture (fusion) reactions $^{32}$S+$^{90,94,96}$Zr,
$^{36}$S+$^{90,96}$Zr, $^{40}$Ca+$^{90,94,96}$Zr, and $^{48}$Ca+$^{90,96}$Zr
with positive and negative $Q$-values for neutron transfer are studied within
the quantum diffusion approach and the universal fusion function
representation. For these systems, the s-wave capture probabilities are
extracted from the experimental excitation functions and are also analyzed.
Different effects of the positive $Q_{xn}$-value neutron transfer in the fusion
enhancement are revealed in the relatively close reactions
$^{32}$S+$^{94,96}$Zr and $^{40}$Ca+$^{94,96}$Zr.
