what type of gauge is it? does it go any higher than 20psi when hot when rpm is increased. The problem with fitting a gauge where the turbo feed is, is that you are effectively bleeding off pressure with oil going to the turbo, so you can get a lower reading. You could try removing the oil cooler as a test, as these have been known to drop the pressure/flow. It also depends on the bearing clerances. If the crank has been ground for large clerances, the pressure will be less but you actually have more oil flow. This is how my engine was built under recomondations from Karl years ago. It never really saw more than 45psi when hot. You could increase this by fitting a washer in release valve spring, but it's not really benificial.