Mercury Racing 25W-50 4-Stroke Synthetic Blend Marine Engine Oil Gallon. SAE 25W-50 synthetic blend oil is specially formulated to maintain viscosity and resist breakdown in extreme operating conditions, including running the engine at high rpm and high temperatures. It also protects against varnish buildup and sludge formation. High Performance engine oil is recommended for use in all Mercury Racing 4-stroke outboards, FormulaRace engines, 525 EFI through 1650 sterndrives, and Mercury Verado outboards in applications where 25W-50 is called for. It meets or exceeds all marine engine manufacturer warranty requirements and recommendations for use of an FC-W marine engine oil.