It is normal, and useful, with a real gauge, but it also confuses the uninitiated. My F-150 has an "idiot gauge" that is not a real gauge. As long as the oil pressure is above a certain, not very high, level the "gauge" reads in the middle. The dealer said that Ford received so many questions from new drivers that didn't understand how oil pressure fluctuates that they got rid of the real gauge. When the oil is cold the pressure will be higher because the oil is thicker. When the oil is very hot it will be lower. The engine water temperature, which is what the Temp gauge is measuring, will reach normal temperatures before the oil is completely warmed up. You can use this to let you know when it is OK to use full power. If you are towing, or climbing, and the oil pressure starts to get low you know that the oil is getting hot, and thin, and you might want to pull over for a while. This used to be common knowledge for us old guys. I suspect the Tundra will never have this problem with its oil coolers.