The magnets normally used are Neodymium, nearly the most powerful nonelectric magnet type. They are the kind of magnet used in computer hard drives, often coming in pairs held just a few millimeters apart with the back end of the hard drive head assembly (the part being made of coiled wire) in between. If you can find a couple of old hard drives - try the local computer junk store - you ought to be able to disassemble them and take the magnets out to stick to your own oil filter. John Nicholas Sarris, a reader of my site, suggested this and provided the following photos as an example.
The same magnets holding themselves to my hand. I could have them stick to each other through my palm, but it was hard to take a good picture. This actually hurt my fingers a bit. As you can see they are strong despite being only 2mm thick. The plate they are attached to itself is 3mm thick.
The importance of neodynium magnets
I thought it worth pointing out here what a potential disaster awaits the home tinkerer if you just grab any old magnet and stick it on the outside of your oil filter. Your common or garden ferrous magnet, like those horrible souvenir magnets stuck to your fridge (you know you've got some) are usually made from iron, and thus have a limited life span which in some cases can be as short as 6 or 12 months. During this time they progressively lose thier power. Not enough for that hideous magnetic photo frame to drop off the fridge, but enough to be a problem if it was stuck to your oil filter. Why's that then? Well, come the end of the filters life, just as the magnet is weakening and the collection of metal particles is at it's highest, one good jolt and it could dislodge, and a large collection of metal shavings and filings could detach from the inside of the filter and find its way back into your engine all in one go. That would be bad. So as much as you might like the magnetic photo of granny and the giraffe from Whipsnade zoo to be stuck in a filthy oily place on your car, don't do it.
prashantha venkatasway--source car bible