gam, how much of a drop have you noticed? Are you using the same viscosity oil, and the same brand filter? Are you comparing to the first start up after rebuild, or after the second oil charge after first flush? Are you comparing oil pressure at the same engine temp both times?

Oil pressure is not a "more is better" situation. A dramatic drop is a flag of a potential problem, but a higher pressure number is not an indicator of proper lubrication.