Just make sure na all of those magnets will be the heavy duty type or high strength magnets. If you use the less strength magnets and it was accidentally taken/fall off then you have now a hefty chunk of ferrous materials now loose in your pans.
Cons of having it all sticked to your pans is you have to open up your oil pans in periodic intervals just to check and clean those accumulated ferrous materials, you don't wan't it to be just being collected there in long periods of time waiting for it to be freed from those magnetic flux and a sudden release will overwhelm your oil filtration.

it is best to put na lang in around the Oil Filter or stick to the Drain plug lang.

If there will be some entrepraneural motorheads na magbenta dito nyan...I'll get two pcs.