just thought Id share, glad I changed fluid when I did. Car has 58,120 miles, I wasnt expecting fluid to be as dirty as it was or the amount of gunk in the pan and on magnet. Ive changed ATF in other vehicles with many more miles on them and the fluid and pan were much cleaner than this. So I would suggest if anybody wants to keep their M6 long term, ATF fluid change at 60k mile intervals definitely a worthwhile task.
I wish I changed mine at 40 or 50k instead of when I did at 80k. Just like you found, I had a lot of metal particles on the magnet and the fluid was in rough shape. I used atf multi vehicle max life and have had no issues to date. I have put about 3k miles since the change. I’m with you, I can’t see paying the oem prices.