Hi:
The pressure you described is normal. Minimum is 13 so you are well above that. As far as the fluctuation, that too is normal. In gear the engine runs at its lowest RPM which in turn provides the lowest pressure. In park or neutral, the engine will idle aprox 300 RPS's faster that will increase the oil pressure slightly. At operating speeds, the RPM's are much higher and the oil pressure will increase.
If you are concerned, you could have the oil sending unit replaced. It is located near the oil filter and just requires being disconnected and unscrewed from the engine block and replaced with a new one. That way you will be sure the pressure is good. The old one may be starting to leak and that will cause a change in what the gauge reads.
Let me know if you have other questions.
Joe
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Thursday, February 26th, 2009 AT 6:50 PM