How loose are the clearances? That would cause the low
oil pressure right there....
I wouldn't use the straight weight
oil. If it's not a multi-grade
oil, it will get really thin when it gets warm. The multi-grade oils have additives that allow the oil to keep their viscosity when hot. The higher the second number, the thicker it is when hot...i.e. a 10W-30 would have thinner oil at temperature than a 10W-40.
If you're worried about low oil pressure, try a 20W-50....if you don't wanna run high viscosity oil, then you could swap your oil pump.