I have the Mini approved Oil pressure guage in my car. When I start the car the guage shows the lowest pressure (50). Only after about 20 minutes does it show 100. I checked my oil on the dip stick. It is in the middle to upper middle of the yellow portion of the dipstick. When I call Ask Mini- they said that this was normal and the normal pressure should be 100-120. Why does the guage not register immediately these pressures when I start it up? Is there a problem?

Thanks,
Gary