when my oil pressure dropped i wasnt to bothered as i was taking the engine out anyway when i stripped it i found the big ends to be totally shagged and a scored crank!!!! personally i would neva put a additive in my oil and only eva use mobil1 15 50 motorsport oil. when cold i have just under 60psi of pressure when cruising at 70 with oil temp around 60 deg i have around 45 psi at idle on a well hammered day it sits around 20 psi and picks up at slightest rev. this is on a standard oil pump as well...... personally id say its rebuild time especially with the mileage