Elliott Investment Management scored a victory on Monday in its board fight at Phillips 66 when prominent proxy advisory firm Institutional Shareholder Services (ISS) recommended that investors elect all four of the activist's director nominees.

ISS’s recommendation comes after Glass Lewis, another proxy adviser, backed three of Elliott’s nominees on Saturday. Shareholders will vote at the annual meeting on May 21. Only four of the 14 positions of Phillips 66’s board are up for nomination at the vote.

Elliott Investment Management scored a key endorsement this week in its heated proxy battle at oil refiner Phillips 66, which is nearing a shareholder vote later this month. On Monday, the activist firm received the backing of the prominent proxy adviser Institutional Shareholder Services.