Oil pressure is pretty responsive to temperature. Check the temperature the next time the pressure is low. With idle speed low and temperature high, oil pressure usually will fall in all but the tightest engines. As your bearings wear and develop clearance, you may want to consider a higher viscosity oil like 15W-40. I like Shell Rotella T.
I had an old Mercedes Benz repair manual that said you were OK as long as the engine developed 7 pounds of pressure for each 1000 rpm. But, I must say, I would be very worried to see my pressure drop to 10 psi or less at idle!