i just put a new oil pressure gauge in my 72 skylark and it reads 0 psi at idle and around 20 psi when driving on the streets. is this right? I checked the entire line from the sending unit to the gauge and no leaks found. what could be the problem?

If the gauge is correct, you have a badly worn engine, and cam, rod , and main bearings that should be replaced. The only other thing could be a bad oil pump.

--or a defective gauge.

I would change the gauge first, as long as the motor isn't making any obnoxious noises like rattling, or knocking, and see what happens.