[rotorwrench]

ATF is basically a low viscosity lubricant with additives to allow it to act as both a good hydraulic oil and good friction modifier for clutch plate lock up as well as a corrosion resistant coating for internal parts.

Some folks mix it with acetone to turn it into a fairly decent penetrating oil. The earlier types available from the mid 70s on were mineral based products such as Dexron II and Ford type F. Modern types are mostly synthetic based lubricants.

Running old engines that have set for long periods depends upon how well the engine has been stored and what kind of shape it was in when it was laid up. While ATF is a lubricant, by itself, it's not good at penetating into old sticky piston rings and valves. The Marvel Mystery Oil (MMO) is better for that. A person doesn't have to fill it up. It just needs to be well coated and left to sit for a day or two to do the job.

I would advise against running anything other than regular motor oil during start up. If you put a lot of crud desolvers into a dirty engine then all that crud will start to break free and get drawn up into the oil system as it is. This could be problematic for the main and rod bearings which are the poured babbitt type. The suggestion to remove the oil pan is not a bad idea if there is a lot of crud in there. It would be the best way to insure a clean crud free start up. It also gives an opportunity to check out the condition of things in there.