When i turn the car on the oil pressure reads fine, about halfway (40psi), but as the car warms up eventually the oil pressure goes down to 0 at an idle and about 20psi while in gear, i changed the sending unit and still having the problem, anyone have the solution to this problem?

 

It could be that your pick up tube has fallen off the bottom of your oil pump. To check for this drain the oil and tap on the bottom of your oil pan with a rubber mallet and listen for it. Another possibility is worn bearings if the engine is getting tired, resulting in lower pressure because of the increased clearances. To check for that put in some 20w-50 oil and see if you get better pressure readings. MOST stock lt1s see about 15-20 psi at warm idle and 40 to 45 at wide open throttle. A basic rule of thumb is you should see a 10 psi increase per 1000 rpms. Anything over 6 psi at idle however is within factory specs but not the norm. If you're seeing 0 psi DEFINITELY find a remedy before doing much driving to avoid internal damage. The oil pump itself may need to be replaced..

 

Since you didn't say your engine seized, I'll assume it's a problem with the reading and not the engine itself.

You could try removing the face and try pushing the gauge around, replacing the instrument cluster, or getting an aftermarket gauge. No simple fix for that problem.

 

Check it with a manual guage. If the OP is below 20psi change to a thicker oil and try it. Change your brand of filter sometimes that helps.
If it stays below 10-15lbs count on a rebuild soon even though the lowest is 6psi in the GM manual.

If the pickup fell off(not likely) ya would still have OP sitting in the drive or just driving down the street.