BP has agreed to sell a 65% stake in its Castrol lubricants business to U.S. investment firm Stonepeak for about $6 billion, a significant step in the oil major’s $20 billion divestment plan aimed at cutting debt and boosting returns. The deal, announced on Wednesday, values Castrol at $10.1 billion, and is the British major's most ambitious and successful asset sale so far in its efforts to streamline operations and scale back its renewable energy investments after years of lagging share performance against its rivals.