The 15-20 on a Hot run engine is Low in my opinion.. That little pressure would worry me that the motor or Pump ?? Worn, and not in too good of shape.. It could be either,,, Good pressure with Cold but LOW when Hot.
How is the compression and is it with in specks of each cyl?
The Oil pressure, using a 20/50 oil and not over 230* in oil temp should be at about 30/40 depending on RPM say , 2400 and up... JMO