when i changed the oil with a thicker/higher viscosity, my 69 thunderbird started going up to H when cold and back down to 3/4 when warm. think readings can be dependent on what type of oil you use, and if your vehicle is quart low or not. quart low and 'thin' oil can reduce the pressure and give you a lower than normal (whatever normal is) reading. wouldn't start replacing anything since you may end up replacing a working pressure gauge with a defective/wrong style.