I had a similar problem on a 2006 Plus 4. Oil Pressure Gauge would wander up to the top of the scale and sometimes off it in a random fashion. The previous owner said it had been like that since new, he had it checked out by his dealer but was told it was fine and nothing to worry about - and as the engine ran fine he let it be.
I wasn't entirely happy and changing the oil pressure sender unit cured it on my car.
