If a small metal particle is suspended in oil
the viscous surface tension of the oil to metal,
is probably high enough (even hot) to not let the
particle change trajectory towards a side magnet
as it zips by in the rushing oil stream.
I do believe the 'reservoir's (sump) of oil moving more
slowly would at least let gravity take over and settle
to the bottom.
Even a rare earth magnet (read as...super strong) used
in the drain plug, is not going to magnetize a tranny case
to any extent that would attract a metal particle. The
reason being 'where' the magnetic field lines lay. If anything,
a round pole magnet like those used in the drain plug, would
most like be poled ,...end to end.
So the magnetic field is being radiated out, vertically, through
the case. Just front and rear of the drain plug where the magnet
would be would both be exposed to the same polar field in
orientation and strength. The thickness, of the case, would be
the only polar variation.
So... does that mean...if anything... that the case 'could' possibly
be magnetized... inside to outside?
Now... since the case curves up on its sides...the 'surface'
of the tranny case in now running 'parallel with the magnetic
field lines. Try stacking a few toy bar magnets in that config.
... a box made of four magnets...yes.... try laying that last one
down, as if it's a drain plug at the bottom of the box.