Mercury - Oil Pressure Gauge Dual Scale Gray Face 2-1/8" (52 mm) 79-8M0065974. Overview. The Mercury Oil Pressure Gauge (79-8M0065974) gives you an accurate, real-time reading of your engine's oil pressure so you can stay ahead of maintenance and protect your motor. It features a dual-scale display that shows both PSI and Bar, letting you monitor pressure in either English or Metric units. The gray face, white numerals, and glass lens provide a clean, modern look that matches the rest of Mercury