Brazil’s state oil major Petrobras swung to a net loss in the second quarter, attributing it to unfavorable exchange rates and a one-off tax charge. The net result came in at some $470 million but excluding the tax charge, Petrobras said, it would have posted a net profit of $5.4 billion. The company, however, presented the tax charge in a positive light. “The main events were the exchange rate variation for the period - an effect between companies in the Petrobras System that has no cash effect or even equity effect - and the impact…