Oil pressure drops off when idling under heavy load

You probably have worn bearings/oil pump. Letting it "idle under heavy load" sounds like abuse to me, especially if the idle speed gets low enough for the light to come on. Seems to me you shouldn't allow that to happen. H

How many miles on your truck? The design of a diesel engine guarantees that it will use some oil. The parts have much more mass than a gasser. Because of this, the engine comes to temperature more slowly. As a result, the engine does not come within operating tolerances as quickly as the gasser. This means that you are bound to get some crankcase dilution. A short trip that does not allow the engine to come to full temperature but for a short time interval, does not allow the volatile to be cooked out of the oil. If the engine is not consuming oil, there is a reasonable explanation for it. The odds are that fuel is bypassing the rings and diluting your crankcase. DO AN OIL ANALYSIS. If you don't wish to do an oil analysis, there is another way to check for dilution. Keep in mind that this method is far from accurate. However, if heavy dilution is present, you will be able to tell. Get a piece of a corrugated brown paper box without print on it. Place a drop of new oil on the cardboard. A couple inches away, place an equal sized drop from the crankcase. Watch to see how quickly the oil is absorbed and spreads on the cardboard. If the crankcase oil spreads quicker and wider than the new oil, you have a fuel dilution problem.