I have a 97 jeep cherokee 4.0*
When I purchased it a few months ago the oil gauge didn't work. It kind of floats around and after about 20 mph drops to zero. The PO said it was the oil sending unit. I sense have replaced that but the issue continues. Any other ideas on what could be wrong?

 

Check with a mechanical gauge to rule out actual oil pressure problems. Next thing I'd check is the wiring/connector for the sending unit and the block and chassis grounds.

 

...and you believed him....