The stock guage reads either zero or a little above mid-scale. Any other reading is a sign of voltage or grounding problems.

If the guage is dropping low (but above zero) at idle, you can expect perhaps a weak alternator or bad grounds. There is a decent chance the engine is OK.

If the guage is dropping to zero at idle, the oil pressure probably really is too low (sub 7 psi) You can try putting a real guage on it, and hope the sender has gone bad, but don't bet the farm on it.

Low pressure at idle is a sign of a worn oil pump, worn main and rod bearings, thin or fuel diluted oil, or a combination of the above. The pressure comes up with RPMs because the oil pump volume is directly proportional to RPM. The more the available volume, the more the pressure.