I just looked in my ancient 1973 Ford Shop Manual. For a 302, at operating temperature the expected oil pressure is to be 40 to 60 psig, the 351 is 45 to 65 psig. These pressure readings are at 2000 RPM. This particular manual doesn't specify an idle pressure though any Ford engine I have put together will run at the higher end of that scale (60 to 65psig), with 20 to 25 psig @750+/- RPM (mild camshaft). Below those figures there is something amiss - sender or gauge, oil filter, oil pump, crank/rod or cam bearings, plugged galleys to name a few potential trouble spots.