In the old days, ATF had more additives, detergents and dispersants as compared to engine oil...
Plus, when oil choice was the heavier the better... ATF thinned things down more, allowing oil to get into more passages and do better cleaning just by virtue of flow.
Nowadays, ATF has less detergent and dispersants than oil... though perhaps it does have a better basestock...
Using a well-formulated oil is the best bet... Use a HDEO if you want extra additive. I believe that mobil delvac 1300 is a group II+ with moly, and a bit of group I mixed in for enhanced cleaning properties.
JMH