Tesoro Corp. said it swung to a $51 million profit from a year-earlier loss during the first quarter, helped in part by improved margins at its Hawai'i and six other refineries.

The San Antonio company said earning more off each barrel of oil refined along with lower operating costs helped it produce a profit compared with a loss of $82 million a year earlier.

Tesoro has been working since late 2007 to make its refineries here and elsewhere more efficient. This includes changes that allow its refinery at Campbell Industrial Park to process heavier crude oil in addition to the sometimes more expensive light sweet crude from Asia that had been refined.

"Even though the start to 2009 has been better than expected, we will continue to pursue our program to reduce our cash breakeven," Bruce Smith, Tesoro chairman, president and chief executive officer, said in a press statement.

"A large piece of that program is intended to increase our margin capture realizations, and, in the first quarter, they were above their average historical levels."

Hawai'i contributed to the improved results, with the refinery here producing a $56 million gross refining margin compared with a $9 million loss last year. Gross refining margin is revenue minus the cost of crude oil, purchased refined products, transportation and distribution.

The local refinery's margin per barrel was $8.53, compared with a loss of $1.49 a barrel last year. For Tesoro as a whole, the gross refining margin per barrel was $12.14.

The refiner reported the Hawai'i operation processed an average of 73,000 barrels daily as opposed to the 67,000 a year earlier, when it was working on improvements and sustained a temporary outage.

Tesoro's overall performance translated into a profit of 37 cents per share. That was lower than the 41 cents a share profit estimation of analysts surveyed by Bloom-berg LP.

Tesoro reported its earnings after the close of regular trading, in which its shares rose 65 cents to $18.45 each.