fork oil dampens the fork travel, by being squeezed through a little hole to allow the fork to move, the thinner the oil, the faster it can make it through the hole and the faster the fork can move for a given force. thicker fork oil will not necessarily "improve" handling, it won't keep the fork from bottoming out during braking or cornering either, as those are long sustained loads and rely on the resistance of the spring to keep the wheel within suspension travel. thicker fork oil will make for a bumpier ride, as little cracks in the pavement or stones etc, will move the wheel too fast for a thick oil to allow absorbtion of the motion, and it will travel through the forks into the frame. thinner fork oil will decrease dampening and can cause the bike suspension to "vibrate", like when you hit a bump, the spring will allow the wheel to move up, and with insufficient dampening, its momentum allows it to carry up further, store energy in the spring, and travel down further than it should, and repeat this until the energy of the bump is dissipated... as far as atf fluid... no clue on using that on forks, it's an oil that is designed to increase in viscosity in regions that undergo mechanical stress, like in the torque converter of a transmission, what it will do with fork performance, i'm not sure
