Just a thought on the ATF thing. I am no oil expert but my understanding of detergent oils v 'straight' oils is that the detergent oil carries a large amount of the carbon etc in suspension [certaily true of my transmission oil at present!!] whereas non detergent oils dont and the gunge sinks to the bottom of the sump.
Most if not all engine oils used nowadays will be detergent oils [i am guessing here] and so the engine internals will be cleaner - might not look it - than a non detergent oil, but nevertheless if a high[er] detergent oil is introduced it will start to clean up the internals even more and carry more of the gunge in the oil. This means that the possibility of the oil filter becoming blocked and bypassing itself is something to be considered ??? - if you, say, run the engine between oil changes with a quart or so ATF in it to clean things up on a high mileage engine.
I am old enough to remember the switch to detergent aviation oils from straight aviation oils and certainly blocked filters were an issue when swapping to a detergent oil after long time useage of a non detergent oil. The ATF additive thing wont be as dramatic as the aviation experience but the same principals will probably apply.
Which by the way is why engine oil goes black very shortly after putting clean stuff in, the carbon etc is being suspended in the oil, not layin in the sump.