No offense, but does this guy do major maintaninace, too? Any mechanic who would diagnose main bearings from an idot light and non-descript noise needs to go back to school. It is probably the wiring harness. After checking the wire to the sending unit, making sure it isn't grounding to anything, doesn't have any chaffed spots, and is properly connected, then preasure test the lubrication system. Jst remove the sending unit and connect a mechaniical oil peasur guage. The preasure should go up when you accelerate the engine. If it doesn't, the oil pump or sump may be at fault. If you have at least 15 lbs of oil preasure at hot idle, the light should stay off. Same for running or under load, but 15 lbs is not enough to lubricate any engine over about 12oo rpm.