oil pressure drop when hot..
The "acceptable" rule of thumb is 10# of oil pressure for every 1,000 RPM. If the engine idles @ 500 RPM @ 5PSI , it will live. Before you do anything, what type gauge do you have? A mechanical can get air in it that will cause a low reading. If you bleed the air out while it is running the pressure will come up several PSI. It is normal for the pressure to drop as the oil warms, though 5PSI seems low. The FE engines seem to like heavier oil as opposed to the newer engines. I use 10-40 in my 150,000 mi 352. It's pressure is 45PSI on start up ,and comes down to 25PSI @ idle @ 188 degrees.