  In sufficiently clean metals, it is possible for electrons to collectively
flow as a viscous fluid at finite temperature. These viscous effects have been
predicted to give a notable magnetoresistance, but whether the
magnetoresistance is positive or negative has been debated. We argue that
regardless of the strength of inhomogeneity, bulk magnetoresistance is always
positive in the hydrodynamic regime. We also compute transport in weakly
inhomogeneous metals across the ballistic-to-hydrodynamic crossover, where we
also find positive magnetoresistance. The non-monotonic temperature dependence
of resistivity in this regime (a bulk Gurzhi effect) rapidly disappears upon
turning on any finite magnetic field, suggesting that magnetotransport is a
simple test for viscous effects in bulk transport, including at the onset of
the hydrodynamic regime.
