My own "testing"...
I have never had a bad experience with any M1 product, and I used them exclusively for a good 10 years. That includes M1 motor oil, ATF and gear lubes. My truck is currently filled with Mobil 1 ATF and has been for some time, works great.
The only M1 product that I find semi-questionable is their grease, horrible with oil separation. I realize that is desirable in some applications, just not in any of mine.
The only real question with M1, IMO, is if it still worth the cost.