  We review the fundamentals and highlight the differences between some
commonly used definitions for the PPN gamma parameter ($\gamma$) and the
gravitational slip ($\eta$). Here we stress the usefulness of a gamma-like
parameter used by Berry and Gair ($\gamma_{\scriptscriptstyle \Sigma}$) that
parametrizes the bending of light and the Shapiro time delay in situations in
which the standard $\gamma$ cannot be promptly used. First we apply our
considerations to two well known cases, but for which some conflicting results
can be found: massive Brans-Dicke gravity and $f(R)$ gravity (both the metric
and the Palatini versions). Although the slip parameter is always well defined,
it has in general no direct relation to either light deflection or the Shapiro
time delay, hence care should be taken on imposing the PPN $\gamma$ bounds on
the slip. We stress that, for any system with a well posed Newtonian limit,
Palatini $f(R)$ theories always have $\gamma = 1$; while metric $f(R)$ theories
can only have two values: either 1 or 1/2. The extension towards Horndeski
gravity shows no qualitative surprises, and $\gamma_{\scriptscriptstyle
\Sigma}$ is a constant in this context (only assuming that the Horndeski
potentials can be approximated by analytical functions). This implies that a
precise study on the bending of light for different impact parameters can in
principle be used to rule out the complete Horndeski action as an action for
gravity. Also, we comment on the consequences for $\gamma$ inferences at
external galaxies.
