I have an 1989 Camaro RS with a 305. I checked the oil pressure with a manual gauge, and it had a little over 20 psi at idle, put in a new sensor and the gauge works but sometimes has really low pressure at idle after a few minutes of driving. Also when this happens can sometimes have a low Idle as well. Any thoughts? Also I would like to mention its the first time the car has been on the road in almost 5 years.

 

If the oil is also 5 years old, change it for something that you can be sure is clean, uncontaminated with moisture, and hasn't lost any fortifiers and polymers.

 

Change the oil.

Also Small block Chev oil pressure requirement is 10 lbs/1000 RPM, so if you're seeing 20 pounds at idle, that is over twice the necessary pressure. It is fairly normal pressure for a warmed up SBC, but it's over twice the requirement. The oil pressure is sufficient. But change the oil anyway.