i have a 91 z28. when i start it in the morning the oil pressure it at 30 psi around. then like 2 mins later when warm or tthe oil preassure goes down to 0. it looks like it goes to -0. it goes below red. no matter if its at idle or rolling. i could kinda hear a little knock. my filter is good and im not burning oil. or leaking oil. is this my oil pump? thx

 

First I'd suspect the oil pressure sender, but if it really is low oil pressure then it's the engine, not the oil pump.