As above, verify your hot/cold/idle/2000 rpm pressures with a master gauge. Hot low idle can be in the 5-8 psi range, with 40 at high idle being acceptable.
Several things affect oil pressure: quality and type of lube, contamination, but mostly just engine wear. The seven main bearing six is very rigid on the big end, which gives it it's robust reputation. Despite that, like any engine, they can suffer bearing and journal wear resulting in lost or weak oil pressure.