Alfa Romeo Repair: spider oil pressure, oil pressure gauge, mile intervals

Question
I have an 89 spider quad with 109k miles. I run 0w40 synthetic oil and the oil pressure gauge has always indicated aprox. 30 psi at idle to 58 psi at speed.  Suddenly, once engine has warmed up, pressure at idle has dropped off to barely on the dial, and only 45 to 50 psi at high rpm.  

Oil is always kept topped off and changed at 3000 mile intervals and there are no unusual noises or temperature indications, but the low readings make me very uncomfortable.

Any obvious things to check, or quirks of the Alfa 2 liter come to mind before I pull the oil pump?

Answer
I am finding that the car guys are getting overenthusiastic about how thin an oil can be used.  You need a thicker oil because of your driving habits and may have prematurely worn main bearings. However, 50 psi at 3000 rpm is fine and 6 psi at idle is ok. Try switching to 5w-50 oil or if you are in a cold climate use 5w-30 in the winter and 10w-40 in the summer.
If ambient temp is always above 60F you can try a 20w-50 oil.