My '88 Firebird has been having the same problems lately. Oil pressure will drop to near zero or below, once heated up to normal operating temp. I took it to the shop and they found normal oil pressure readings with a mechanical gauge. They replaced the sending unit and it didn't fix the problem. Runs normal, no noises or knocking. Any ideas?? I'm guessing it's the gauge itself.

 

It was my gauge. I had swapped instrument clusters because one had a bad odometer, and that gauge just read low. When I swapped the old gauge into the new cluster, it was all good.

 

As a follow up to my follow up, it was the gauge. After almost 3 weeks, the pressure never drops below 15psi. I guess they just go goofy at some point. It's not that bad to swap them out and I had a spare, but I would go junkyard hunting for another before I did anything serious there. I ended up getting a new sending unit installed for nothing.