Reuters: Researchers weighed in on the controversial question of how much oil poured into the Gulf of Mexico during the BP Plc spill and said on Thursday the total amount was around 4.4 million barrels. The figure, derived using a technique called optical plume velocimetry, represents the first independent peer-reviewed study of the worst oil spill in U.S. history and conforms closely to the most recent estimate by government scientists. The spill began in April when a deepwater rig …