Yeah the additive is called "Lubrizol"

good luck finding it however most people seem to think modern dex3 fluids have enough additives to do the same function.

v8fordman - the fluid doesn't affect this modification one little bit. Put your car in gear then wobble the gearstick left and right and THAT is the slack that dissapears when you remove the rubber coupling. It also stops the up/down slack too which is what makes the gearchanges much nicer, notchy and direct.