Just an FYI-The oil pressure gauge is not an indication of the amount of oil that is in the crankcase, it's the amount of oil pressure produced by the oil pump. You could have a quart of oil in the engine, and the pump would still be putting out the same amount of pressure as long as oil was available.
Having said that, the oil pressure gauge could be going south, or a different viscosity/type of oil used. As the change is very nominal, no need to go into any mechanical mumbo-jumbo.
It's fine, but just keep an eye on the gauge.