I'm personally not a big fan of "add-on" junk as it's pretty much snake oil. If you don't think your filtration is up to task as designed by the manufacturer then you need to seriously look at some hard-core racing or industrial filtration. I've used nothing but OEM-quality filters then the Mobil-1 synthetic filters when they became available and have never had any filtration issues, and most of my Isuzus get well over 150K miles with little if any attention other than routine maintenance.

If magnets turn you on though, then look to filters built from the outset to incorporate magnets in their structure where they could maximize their effect on filtered fluid. Several have been posted here over the years so should show up in a general search.

Magnetic drain plugs can be purchased at big-box auto stores like AutoZone, PepBoys etc. and at NAPA as well. I'm a big fan of the Fumoto Engine Oil Drain Valve as it makes changing oil a snap, requiring no tools, especially on my heavy equipment where it's not so easy to get a wrench on the drain plug. Can't say enough wonderful things about the Fumoto!