Melling is the leader in oil pump engineering, prototyping, and production. Melling specializes in several different styles of oil pumps including: Variable Displacement Oil Pumps, Vane Oil Pumps, Cast Iron Oil Pumps, Gerotor Oil Pumps, Aluminum Oil Pumps, Crescent Gear Oil Pumps, and traditional Spur Gear Oil Pumps. GMC, Mercury Marine - 366,396,402,427,454,482,502. Big Block GM Oil Pump.