Shares of Tesoro Corp., the oil refiner that operates Hawai'i's largest refinery, fell to their lowest level in almost three years after Oppenheimer & Co. cut its profit forecast on the company, Bloomberg News reported.

The news service reported Oppenheimer analyst Fadel Gheit wrote that Tesoro's refining margins are being squeezed by high crude oil and feedstock costs and weak demand and lowered his profit estimate by 47 percent to $2.07 for 2008.

Shares of the San Antonio-based company fell 5.5 percent to $21.95, the lowest since May 2005.