[James]

I don't know how accurate this is, but I generally use 10 psi per 1000 rpm as a base line. If idling at 500 rpm with 5 psi then it is OK. If it is idling a 1000 rpm then I would look deeper into the engine. I would drop the oil pan and check the bearing (rod and main) clearances.

This would also depend on the oil you use. A multi-viscosity (like 10W30) then your oil pressure would be lower. A straight weight oil (30W30) would be higher. Also oil that getting close to an oil change would tend to be lower too.

Someone else might have more or better information on this.