Last December, Sunoco said it had confirmed a number of unsolicited inquiries into acquiring the Tulsa refinery, which has 375 employees. The facility can process up to 85,000 b/d of crude oil into fuels.

The company is investing in a clean fuels upgrade project at the refinery, at an estimated capital cost of $400 million. It will convert high-sulfur distillate into low-sulfur diesel. The project includes installing a new 24,000 b/d hydrotreating unit.

Tulsa is the last of three base oil refineries Sunoco once operated. In 1992, it closed its base oil plant at Marcus Hook, Pa., which had capacity to make 10,000 b/d but required major capital investment for upgrades. A second refinery, in Yabucoa, Puerto Rico, had 9,200 b/d of base oil capacity and was one of the first to produce Group II quality stocks. But Yabucoa stopped processing crude in 2000, and the next year Sunoco ended lube operations there for good and sold the refinery to Shell Chemical.

Sunoco also has dwindling internal need for base oils. In the mid-1990s, its lubricants division was not profitable, and it acquired the Amalie and Kendall motor oil brands from Witco in an attempt to build volume and create operating efficiencies. The Amalie name was resold, but keeping Kendall allowed Sunoco to lay claim to nearly 5 percent of the U.S. finished lubes market.

In 2000 though, the same corporate streamlining that prompted the divestment of the Yabucoa refinery also led Sunoco to put its lube marketing assets, including the Kendall name, on the block. In 2001, Kendall was sold to Tosco (now owned by ConocoPhillips). The company then closed its three blending plants, at Tulsa, Marcus Hook, Pa., and Richmond, Calif., further loosening its ties to lubes.

Lisa Tocci contributed to this report.