My thoughts on this is you probably have a HV
oil pump and that is scavenging the pan too quickly, dropping the
oil pressure until it returns to the pan. I had a similar issue until I replaced the HV
oil pump with a stock oil pump. My oil pressure went from 60 psi to 50 psi on startup, and now remains at 45 psi hot. You can add a couple extra quarts, but that's just a band-aid. I've found that these FE engines can be quirky at times. Good Luck.
PS. The oil pump only took an hour to replace.