Oil pressure light coming on at idle

I have a high mileage 67 engine in my Valiant. It runs great but when warm, the oil light flickers at idle with the AC on. It had a manual gauge in the car I took if from and the idle pressure does drop below 9 psi when hot. I have another engine ready to go in if it ever dies, but for now I've been adding motor honey (STP) to up the oil viscosity whenever it happens. It's a Band-Aid approach, but it will make the pressure go up and the light go off.