Jackaroo 3.5L V6 oil pressure

My 2000 model 3.5L V6 Jackaroo is suffering from fluctuations in oil pressure. At normal operating temperature and normal driving speeds (say in the 2,000 to 2,500 RPM range), the oil pressure can differ from trip to trip. The motor has 110,000 kms on it and from new until recently the oil pressure gauge always sat neatly on the '6' marking. Although the gauge doesnt have many markings on it, I am estimating that the oil pressure at the same RPM and full operating temperature is now varying between about 4 and 7 (i.e. sometimes lower and sometimes higher than in the past) with it still registering a steady '6' on many trips.

The local Holden dealer hasnt been too helpful in the past, hence I am interested if anyone has experienced the same and can give me some advice on this one. As the pressure is fluctuating higher and lower trip to trip than the past norm, I am guessing it may be a fault with the pressure sensor/sender or a sticky pressure release valve.