First, stop using the STP. Normally, additives should be used only as a last resort, not a matter of routine. That's probably giving you that really high cold PSI.
Second, use an oil with a higher viscosity index. You'll get less pressure drop with temperature increase. You should be able to get the pressure you want with Rotella T6 5W-40, which has a viscosity index just under 170, or Mobil1 15W-50, VI of 160. They will thin less, and consequently hold pressure better than oils with lower VIs. Both also have ZDDP levels above 1200 PPM, which is a good anti-wear package for stiff valve springs. They are synthetics, which will cost a little more, but both are readily available, and often on sale in 5 quart containers at Wally World.
