I assume you mean to say: "Give it a little gas and the gauge drops to zero". How does it do under breaking?
If the
oil pickup is in the front of the pan and the
oil goes to the back of the pan on accelleration, THATS why your seeing zero. What does the pressure do under heavy breaking?
Another possibility is your
oil level in the pan is "low", no matter what the dip stick indicates.
Regardless of any of the above, 20-40 lbs pressure hot doesn't sound right. As stated above, confirm the gauge reading.
Clogged up pickup screen?