Gas station and convenience store operator United Oil of Gardena was purchased by New York-based asset management firm Fortress Investment Group for an undisclosed sum last week.

Fortress picked up more than 130 United Oil locations throughout Southern California as well as a distribution business that delivers fuel to some 100 Shell, ConocoPhillips and Valero stations.

With a staff of about 730 and revenue of roughly $3 billion in 2012, the most recent data available, United Oil ranked fifth on the Business Journal’s list of largest private companies.

United Oil was established in 1955, and the deal was struck with the son of the family-run company’s founder. Company President Ron Appel, whose father started the business, and son Jeff Appel, its vice president, left the business immediately after the deal closed. Fortress has installed Joe Juliano, former group president at SC Fuels of Orange, as chief executive.

The sale also included Apro, an operator of gas stations. Apro was owned jointly by United Oil and South Gate’s World Oil Corp., which operates a separate line of gas stations and is owned by brothers Robert and Steven Roth.

Representatives of United Oil, World Oil and Fortress declined to comment.

United Oil is known for its splashy architecture. One station on La Brea Avenue in Los Angeles features a ramp that takes vehicles on top of the convenience store to a carwash and another was designed with a Western theme with antique barrels and cigar-store American Indian statues.

Jeff Appel also pushed for the installation of Gas Station TV, a video network installed at the pump, and was an early adopter of the technology – first installing it in 2007.

Fortress, which has an office in Century City, has about $63 billion in assets under management. The Union Oil transaction brings the retailer under Fortress’ $13 billion credit business, which includes hedge funds and private equity investments.