[The Newell]

I can think of several of things it might be - weak oil pump, bad/clogged oil filter, foaming or frothing of the oil (bubbles don't have much pressure), water (from radiator) in the oil, bad gauge, bad pressure sending unit, or an electrical problem that affects the gauge or sending unit (they depend on 12v power, good ground, etc). You kind of have to use a process of elimination. Changing the filter is an easy step and so is changing the oil, though it seems unlikely the oil is contaminated.