Editor’s Note: This story has been updated to correctly report the relationships of the companies involved in the transaction.

United Oil, the Gardena-based gas station and convenience store chain, has been sold to the asset management firm Fortress Investment Group for an undisclosed price.

United Oil has operated some 130 gas stations in the Los Angeles and San Diego areas. The company was founded in 1955 and the Business Journal ranks it as L.A.’s fifth-largest private company, with revenue of more than $3 billion.

Two companies, United El Segundo and Apro, did business as United Oil. The ownership of Apro was split evenly between United El Segundo and World Oil Corp. of South Gate. World Oil will continue to operate a separate network of gas stations and other operations within the petroleum industry.

Apro was also sold to Fortress Investment Group.

United Oil’s employees were retained in the deal, although Fortress Investment Group will select new senior managers.

Fortress Investment Group is headquartered in New York and has an office in Century City. The firm had $63 billion worth of assets under management as of March 31.

Shares of Fortress Investment Group fell 25 cents, or 3 percent, to $7.23 Tuesday on the New York Stock Exchange.