What is the difference between the Original and Slim formulas?

Not much. Neither are "better" than the other. Both are the same exact formulas that will produce the same long lasting results, the only difference is the Original has thickener in it. The reason is for this is it makes it easier to brush. The Slim does not have the thickener, and is great for rolling on and spraying. Ordinarily you wouldn't have to reduce it, but if you do, start with 5% reduction with MCT101 or medium urethane reducer and use enough to get it through your gun.