U.S. oil prices slid below the psychologically key $80 a barrel mark Friday after the Bank of Japan unexpectedly expanded its stimulus measures, boosting the dollar. The Bank of Japan announced additional stimulus measures very early Friday. The yen fell on the news, while the dollar rose against a basket of currencies. Oil is traded in dollars, so a stronger dollar makes oil more expensive to buyers using foreign currencies. The dollar’s recent strength has added to the concerns already weighing on oil prices, including ample global supplies and lower-than-expected demand growth. Prices have tumbled more than 25% from mid-June highs, and some analysts are calling for further declines in 2015. Light, sweet oil for December delivery recently traded down $1.40, or 1.7%, to $79.72 a barrel on the New York Mercantile Exchange, near more-than-two-year intraday lows. U.S. oil prices have dipped below $80 a barrel […]