This weekend wired and plumbed the sending units for three new gauges: oil temp, water temp, and oil pressure. I was somewhat surprised that the water temperature rose to between 150 and 180 before the oil temperature moved off the peg at 140, but whatever. What really confused me, though, is that the oil pressure in a cold engine is around 70 psi; as the oil temperature rises the pressure drops. Once the oil reaches around 180 the pressure settles at 35 psi, which is where I expected it to be. I recall from 50 years ago that Charles' Law (I know, I know, it's for gasses) holds that the volume of a gas rises with its temperature. Since the oil system is closed, meaning the volume is constant, I would have expected the oil pressure to rise as the temperature rose (Boyle's Law? also for gasses). Will someone explain this to me like I'm three years old? Why is this so counterintuitive? Thanks!