If the warning chime sounds, when it goes to zero, the sender is likely still bad. We have changed three (OEM sensors) to get one that works properly. If no chime goes off, I would suspect the gauge. GM has had a great deal of trouble with this as well. The gauge can be repaired, by shops that do instrument panel work.
We have also seen some GM engines actually loosing oil pressure at fairly low mileage (60-100K.) There is an adaptor that allows oil pressure to be checked easily. Find someone with one of these adaptors and have them verify the oil pressure, if the above does not help.