BP completed the sale of a 65% stake in its Castrol lubricants division to U.S. investment firm Stonepeak Partners. The sale was valued at $6 billion.

BP said it will use the proceeds to pay down debt and strengthen its balance sheet, a priority in its financial restructuring. The transaction helped BP reach its net debt target ahead of schedule. Net debt fell to $13.9 billion.

The sale is part of BP’s strategy to streamline operations and focus on core businesses. BP will retain a 35% stake in Castrol, giving it a share in the brand’s future growth.

Castrol will operate with a new board chaired by former Shell CEO Ben van Beurden. The new structure gives the business greater independence to pursue its strategic priorities and accelerate growth.