2001 Jeep Cherokee Engine Oil Pressure

I just bought this car with 100291 miles. Overall it a seems to be ok. But today the light of Check Gages turned on. When the car cooled down I check it and found the engine coolant bottle was empty. I filled with water and drive it again and just when I arrive to my house the check gages light turned on. Once again the engine coolant bottle was empty. A mechanic checked the car it wasn't dripping the coolant on the floor, checked coolant pressure with a pressure pomp and found it ok, check the carpet floor of the passenger and it was dry, but when turned on the car noticed that the engine pressure oil indicate low levels. When the car is on but stop it reads 10, when I run like 30 to 40 miles it reads 20 and when I run 50 miles reads 30. It's the readings normals? In the owner's manual said that in city driving should read 20-60. I know there must be something wrong. What should I do?