Okie,
I wouldn't run that motor any more until you can get it checked out.
I'd be leary of anything under 20-25 psi at hot idle, and you generally want 10 psi per 1000 RPM at speed. IE: 50 psi at 5000 rpm, 60 @ 6000, etc.
The drop after 300 miles indicates something changed in the motor. You could have a bad bearing or it could be the
oil pump picked up a shaving missed during block cleaning.
It could also be as simple as a stuck relief valve in the
oil pump.
Just have it checked before more severe damage is done.
Good luck,
David