Rocky:
21 pounds at idle is normal. I am assuming that when you say the oil pressure drops from 48 to 21 pounds, you are going from a higher RPM to the stopped idling RPM. If that is the case, this is normal. Your oil pressure will build with RPM. Something like 10 pounds per 1000 RPM is typical.
As an additional word of caution, oils matter for your new engine. Follow your mechanics advice. If this is a flat tappet cam engine, you should be using a specialty oil like Brad Penn with higher levels of zinc.
