I'm a former parts store owner, drag racer and machinist and I thought I could figure most things out, but I am honestly stumped. Six months ago my 89 camaro 305 had 55psi oil cold & 40 psi hot. Then, overnight, 30 psi cold and 15 hot and down to near 0 at a stop light. I thought worn mains but, no. Also, doesn't seem to be a function of temperature, but rather of time. It starts after about 20 minutes of run time whether the temp is below 150 or over 200. New oil & filter, no change. No engine noises. No leaks.  Anyone have an idea?