The curie temp for Neodymium (used in a filtermag) is around 600F (or higher, depending on the exact variant). If the magnet were regularly exposed to temps in excess of the curie temp, that would significantly and permanently remove its magnetism in a relatively short period of time. At lesser temperatures you can see *some* loss of magnetism temporarily, which is restored when it cools. I'm not sure how much Nd loses at typical oil temps, but probably not much.
Heat effects aside, if it's just sitting there stuck to some iron (or pulling light fragments of iron from oil), it would take a long time for it to significantly degrade. I'd guess the useful life of a FilterMag to be at least 10 years before you could tell the difference from a fresh one, could be much longer than that even.
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