The floor is controlled by the pump volume, the ceiling is controlled by the relief spring. Sounds to me like you are on the edge of having too many leaks for a HV pump to keep up with. You can increase the pressure by using a heavier oil, but that doesn't actually provide more lubrication for the engine. It just makes the pressure on the gauge higher since the same pump is now trying harder to push the heavy stuff thru the engine.
You might be okay as is. If you want to run a thin oil and push more of it thru the engine then you'll need a higher volume pump such as the Milodon pump.
