I have a X-block stroker in my cuda and it is does about the same thing. Cold 70-80 psi, hot idle 10-20psi. Got me worried also at time . The oil pressure works like a tack and will go from 10 to 70 with the rpms. I do keep the idle real low though. I also notice if i shut down the car for a few minute(it cools off i think) and restart it at idle the pressure will be more on the 20 side. Seems the heat effects it a whole lot. Been driving it for 2 summer like this now and so far no break down. Read alot of post and i hear dont worry about it and i also hear tear it down alot. I chenged guages and sending unit to make sure that wasnt the problem it stayed the same. I would blame the pump myself i'am unsure whats in mine or mileage also but someone told me that it sounds like they threw a high pressure pump in for racing by the way it acts.