Oil pressure light coming on at idle

Like others have stated.....check to see if the oil pressure is actually low or if it's just a faulty sending unit. If the oil is thin, change it and the filter. I suspect that the engine just has a lot of miles on it and needs new bearings. Heavier weigh oil will get you down the road a little while longer but it's no cure for bad bearings. As for viscosity ratings, a lot will depend on your climate and the use for which you put the engine. When it's 40 below, 0-20W might be too thick on a new engine.....when it's 110 in the shade, 20-50 is fine on an engine with some miles on it.