This is a direct quote right out of my tech. manual. This should answer your question and then some...Hope it helps...
Oil Pressure Gauge Interpretation
Cold Start
When and engine starts, all the oil is in the sump and the oil pressure is zero. The pump cannot begin to deliver oil or generate oil pressure until it has sucked up cold oil through the filter screen and the pick up tube. Hence, cold start lubrication is greatly assisted by having a short, wide pick up tube and oil that has very good cold flow properties.
When the oil reaches the pump it is then forced through the filter and then through the engine. However, as the oil is still cold and the oil channels are very narrow, flow is slow and a large back pressure develops so that the pressure Gauge registers a dramatic pressure increase.
As the oil circulates and warms up, it flows faster and the oil pressure declines to a stable level. It is only at this point that the engine is being properly lubricated. Until stable oil pressure is obtained wear rates are high due to inadequate oil delivery to the wearing surfaces. Under cold start conditions, a "good" oil is one which gives stable oil pressure the fastest.
Normal Operation
Under ideal circumstances, oil pressure should be stable and any large increase or decrease should be investigated for possible mechanical damage.