Mercury - Oil Pressure Gauge 0-80 PSI Dual Scale Black Face 2-1/8" (52 mm) 79-8M0069427. Overview. The Mercury Oil Pressure Gauge (79-8M0069427) gives you a clear, real-time reading of your engine's oil pressure so you can keep your motor running safely and efficiently. Featuring a dual-scale display that reads in both PSI and Bar, it's easy to read no matter which units you prefer. The black face, crisp white numerals, and glass lens provide a sharp, professional look that matches other Mercury