I studied that question about motor oil in the AT545 a couple of years ago. There should be some info here if you search. In a nut shell, there was a very specific motor oil approved, and it was supposed to be an improvement. Then it turned out that this "fix" wasn't so hot after all, and they no longer recommend this "trick". And that specific grade and type of oil is hard to find, anyway. So it's back to ATF, or Synthetic ATF if you can afford it.
Keeping it clean and cool is more important.