DON'T do it skydive. I also knew an older Gentleman with a '66 Riviera that swore by running ONLY ATF fluid right before an oil change. He ran it for a couple hundred miles with the ATF in it, and claimed it cleaned the engine beautifully. While this is certainly more extreme than a 1/2 pint, ATF oil is a completely different formulation than the oil used for a crankcase. Although it is high in detergent, it is not meant to provide the same protection under extreme dynamic rotating loads. The last time I saw him his Riv was nowhere in sight. He told me he had spun every connecting rod bearing in the motor, and probably the mains too. The ATF oil certainly cleaned away all his bearing material!
Use the synthetic as dmorrison suggests.