Quality State Oil Co. (QSO), founded in 1957 has offices and bulk petroleum plants in Sheboygan, Two Rivers, and Sturgeon Bay, Wisconsin. QSO distributes Citgo Petroleum and Exxon-Mobile gasoline, distillates, and lubricants to industrial, commercial, agricultural, and heating oil customers. A registered common carrier, QSO owns and operates its own semi-tanker trucks.

QSO also owns and operates 16 convenience stores and also supplies a number of individually owned convenience stores and truck stops with either Citgo or Mobile branded gasoline and diesel.