Ive read before on some engines oil pressure can be as low as 5- 10 psi at idle, as long as it comes up with RPMs. Also Adding some Lucas oil stabilizer will bring the pressure up quite a bit. But that may be just masking the problem. Another thing to check could be a defective or clogged oil filter could give you a low pressure reading, or a worn oil pump,clogged pickup screen, bypass valve hanging open?