Oil light monitors engine oil pressure only. When it comes on, there are any number of potential reasons, could be down 2 quarts of oil or more, the oil could be extremely thin, the sending unit could be bad, the pickup tube in the oil pan could be too close to the pan, or too high, or the pump could be worn out, lastly the bearings could be worn out. Or any combination of the above. Assuming the engine is original and not recently rebuilt, I would expect that the bearings are worn. First step should be to connect a mechanical gauge and get a reading with fresh oil and filter, using a good quality oil, 10w30 minimum grade.
