ORIGINAL: 46u
Does ATF have the moister absorbing properties after run oil has? You do not only want to coat the engine with oil but you want something that absorbs moister. If I did not use ARO I would use Marvel Mystery air tool oil.
I wouldn't want anything that absorbs moisture and keeps it in the engine. I'm not the nobel prize winner of oil knowledge but as far as I know oil and water don't ever mix.
That's why flushing the engine with WD-40 to get rid of any trace of moisture is good. I am not talking a few drops here, I mean flushing it.
After that pretty much any light tool oil or similar will do the job. I've used WD-40 + ATF oil for ages and never got any rust or any other problems ever.