  The present obliquity of Mercury is very low (less than 0.1 degree), which
led previous studies to always adopt a nearly zero obliquity during the
planet's past evolution. However, the initial orientation of Mercury's rotation
axis is unknown and probably much different than today. As a consequence, we
believe that the obliquity could have been significant when the rotation rate
of the planet first encountered spin-orbit resonances. In order to compute the
capture probabilities in resonance for any evolutionary scenario, we present in
full detail the dynamical equations governing the long term evolution of the
spin, including the obliquity contribution.
  The secular spin evolution of Mercury results from tidal interactions with
the Sun, but also from viscous friction at the core-mantle boundary. Here, this
effect is also regarded with particular attention. Previous studies show that a
liquid core enhances drastically the chances of capture in spin-orbit
resonances. We confirm these results for null obliquity, but we find that the
capture probability generally decreases as the obliquity increases. We finally
show that, when core-mantle friction is combined with obliquity evolution, the
spin can evolve into some unexpected configurations as the synchronous or the
1/2 spin-orbit resonance.
