On a cold start the oil is very thick and viscous so the pressure reads, and is, a little higher. As the engine warms up the oil gets thinner and the pressure drops down to normal. An example would be in Canada. My 1999 Tahoe was originally from Manitoba, Canada and it had a piece of metal, with an AC electrical cord that led up towards the battery then 12 ft. coiled up. The cord was to be plugged into an AC electrical outlet. It heated the piece of metal, or blanket as I liked to call it, which in turn keep the oil from getting too viscous or congealed. If the oil gets too thick the oil pump can't supply enough oil to the cylinders at start up. No oil would cause the engine to seize up. Hope this helps. Aloha!!