Let me start by saying that if you go to a tribology website, there will be no category of bearing wear that has to do with low oil pressure, only lack of lubrication. The oil pressure isn't what supports the crankshaft. When a cylinder fires, you have maybe 600-900 psi acting on piston with a surface area of 13 sq inches and it's being supported by a main bearing with a supported surface area of maybe 3 sq inches? The force on the piston creates a downward force of 8000-12000 lbs while the upwards force 30 lbs of oil pressure in the crank journal might be only 90 lbs (it's actually 0 because the oil pressure acts on all sides of the crank, therefore, cancelling itself out). The oil pressure generated by the pump has no contribution to the bearings ability to carry a load, it only ensures that there is more than sufficient oil supply to the bearing. Instead, think of a car hydroplaning. The way a wheel makes contact with the road forms a sort of a wedge infront of the contact patch. If you push that wheel through water at high speed, water gets packed into that wedge, creating tremendous pressure on the wheel. That pressure can actually lift the car off the road and that is when you have lubrication. Inside of your engine, that same wedge forms between the bottom of the crank journal and the bearing shell. As the crank rotates, friction between the crank and the oil pulls the oil down into the wedge much like water is packed infront of the wheel, pressure is generated, the crank is lifted off of the bearing, and lubrication is achieved.

Here comes the tricky part. As engine components reciprocate and vibrate, they may cause rapid pressure changes in the lubricating film. Entrapped gasses may come out of solution, forming bubbles, or the oil may even boil. When these bubbles pass through the high pressure region of the wedge, they collapse so rapidly that they will errode the surface of the bearing. This is cavitation. Putting the oil supply to the bearing under pressure helps keep bubbles from forming. The greater the speed, the greater the pressure differentials, and the greater the oil pressure required. Smokey's rule of thumb was 10 psi per 1000 rpm. Which says you're ok. GM says 5-6 psi at idle minimum for stock engines.

Note: Like my post on cams, these are all of my theories that I developed in the last 15 minutes, especially the cavitation part. Don't take what I say for granted, I may be wrong. But I do know that in mild street engines 13 psi at idle is ok. I ran my engine at 6 psi for over 20,000 miles and kept a close watch on bearing wear (pulled the pan and plastiguaged bearings periodically) and had no excessive wear.