The disc magnet I have specified has a pull force of 54.5 lbs when on a steel plate. I think that an act of God would be required to rip that from an oil pan regardless of where I mount it.
The thought was to keep the magnet on the out side so I cull pull it from the pan when doing oil changes to remove the "captured" metal bits. These are Neodymium magnets at the highest strength possible so are not your refrigerator magnets...
if I have them on the inside I could only clean it when I pull the oil pan. and being that its not a easy 5 min task on our motors to do that the magnets would likely never get cleaned.
I am using the stock oil pan.
Edit: thank you for pointing out my spelling error. should have title and all posts updated.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)