If you are getting magnetic particles in the oil then you have a very
serious problem; a magnetic drain plug will not even begin to address
this. Most of the metal particles that would otherwise accumulate at
the bottom of the oil pan are infact trapped by the oil filter during
normal engine operation. Thus, no magnet is needed on the drain plug.
The filter will trap whatever might otherwise accumulate down there.

The only time I have seen magnetic drain plugs do any good at all is on
a manual shift transmission. Genearally speaking in a manual gearbox
the only way the bearings are lubricated is by the action of the gears
churning through the oil, i.e. no pump, no filter. The drain plug
usually has an internal magnet, and you remove the debris from it when
you change the oil. Generally this is done when you replace the clutch.
The only exception I know of to this rule is the old VW beetle/super
beetle/thing/karmann ghia, et al, which specified a 30,000 mile
interval for replacing the hypoid gear oil.

Chris