Tesoro Corp., the owner of Hawai'i's largest oil refinery, said its directors approved a 2-for-1 stock split and a doubling of the dividend.

The stock split will be effective on or about May 29 for shareholders of record on May 14, the San Antonio-based company said in a statement yesterday. The quarterly dividend will be increased to 10 cents a share from a split-adjusted 5 cents, payable June 15 to shareholders of record at the close of business June 1.

Chief Executive Officer Bruce Smith is expanding Tesoro, already the second-largest refiner in the Western U.S., with its planned acquisition of a Los Angeles-area oil refinery and 390 filling stations from Royal Dutch Shell Plc for $1.91 billion. Only Chevron Corp. has a larger refining capacity in the U.S. West.

The increased dividend "shows confidence in the refining market," and the stock split reflects "the great run they've had, said Jim Byrne, an analyst at BMO Capital Markets in Calgary.

Shares of Tesoro fell $1.18 to $120.02 in New York Stock Exchange composite trading. The stock has risen 82 percent this year, the biggest gain among the 971 members of the Russell 1000 Index.