My BBC shows 70psi of oil pressure when cruising at 4,000rpm. If I floor it, the pressure will drop all the way down to 20psi by the time the motor spins 5,000rpm. According to the gps, the boat is still building speed, but i have to let off the gas because of the drop in oil pressure. If i ease into it, the pressure will hold at 60spi. Its almost like the oil pump is cavatiting. i have a 10-quart pan on the motor, but i dont know if its got a trap door or windage tray. Any suggestions? I was considering going to a thinner oil.