The big oil company that is in the yellow container with a Z in their name went after Amzoil and had them in litigation for years over the use of the Z. At one point they also tried to buy them out but the owner (AMatuZio) refused to sell. Since its seemed to be stacked against them, they trademark the Amsoil name and by time the suit was settled in their favor, the name recognition was to far along to go back.