OPI nail lacquer thinner is made to make thick polishes thin again. The instructions say to "add 2-3 drops of thinner to achieve desired consistency", but I find that I use more than that. It really works for thick polish. The only ingredients of this nail lacquer thinner are Butyl Acetate and Ethyl Acetate which are both also the main ingredient in most nail polishes. I have no idea how this relates to each other though.

Many people probably know that mixing nail polish remover in polish also makes it thinner. The problem with this though is that when applied, the polish will most likely have a milky film over it. OPI nail lacquer thinner does not do this to your polish. Don't ruin your favourite nail polishes, save them with this product instead!