An oil pressure gauge can fluctuate for many reasons, including an actual automotive problem as well as reasons like fluctuation in oil temperatures. In most cases, the gauge will fluctuate to indicate low oil pressure. When the gauge fluctuates to indicate high oil pressure, it is often indicative of a pressure relief valve problem.
Factors that can cause oil pressure to fluctuate lower include high or low oil temperatures, diluted or worn oil, worn engine bearings or a worn oil pump, a damaged oil pan or dropped crankshaft plugs. Another reason for the fluctuation in oil pressure can be simply low levels of oil in the tank.