There is no real oil pressure in the head. Only free fall oil from the rockers. No real pressure in the rocker arms only the head pressure from the weight of the oil from where it enters the head to the top of the rocker arms where it free flows out the top.
Pressure is only built from the oil pump to the filter where it free falls over the lifters to run back into the pan. That pressure is regulated by at ball in the stem of the oil pump. The highest volume of oil from the pump flows by the ball in the stem into the center of the pump to free flow out the center of the cam shaft an into the pan. The only pressure to oil the engines main, cam and rod bearings is from centrifugal force cause by the spinning cam shaft as the oil passes thru the center of the cam to the front.
The oil path shown by the red wire.
If you have a good radiator cap you will have more water pressure in the engine than oil pressure. You would have water in the oil as well as the radiator from a head gasket or a crack in the head or block.
That is just the way this engine is built. It can not be compared to any other engine oil or water mix.
That kinda rules out the head gasket.