The one instrument I would pick would be an I pad loaded with I Fly. If limited to one gauge it would be a compass. Safely navigating to a LZ would be my priority. The eng will run for a considerable time with zero oil pressure.
👍👍 two thumbs up never go without a back up, and an iPad is a great back up. However, oil pressure is my number one concer, I have my eyes, my gut, and piloting skills for airspeed, altemiter, etc.