Oil pressure indicator drops to 0 , jar dash, returns to nromal

indicates a problem oil pressure sender unit
have the oil pressure checked using a mechanical pressure gauge and if OK then replace the sender unit or gauge

Did they change your tire pressure. tire pressure affects the mileage dramatically and when ever I have someone change the oil I tell them not to change my tire pressure.

Well it could be a faulty pressure sensor, or you could really have a problem with the oil pump or engine. As the engine warms up the oil gets thinner and pressure can drop if the engine is worn out.
You can check the oil pressure by using a mechanical gauge and bypass the gauge on the dash.

Most likely a bad sensor. If you had 0 pressure you would know by vehicle reaction. Get an oil pressure gauge and remove sensor and install gauge. What is actual pressure? Bad sensor? Always a good idea to do internal clean, cheapest is SEAFOAM

Pull the sump and check the main bearings for spinning in cap. The knock indicates a bearing and the fluctuating pressure indicates a oil hole being opened and closed as the bearing shell moves over the oil hole in the crank.

Could be the Oil Pressure Sending Unit. I do recall GMC having issues with the instrument cluster on 2003's. Try slapping the dash when the gage reads zero.

The first order of business is to check the oil pressure while the gauge is not working. If the oil pressure is fine and the system reading zero, there is a fault in the oil pressure switch circuit. Often, the problem is related to the oil pressure switch itself. I have seen cases, too, where the oil pressure switch fails internally, causing oil to push past the seal and into the wiring connection causing a short. The wiring harness has a single pin.

Another area, though it fails much less often, is a break in the wiring between the oil pressure sender/switch and the ECM module (located in the right passenger footwell) and between the ECM and the instrument cluster. The instrument cluster may also have a short, though the sender is the most likely issue here.

-Ed
ASE Certified Master Technician since 1984 - Everything from toy cars to the largest of trucks
Consultant for the North Carolina Consumers Council

Cluster is going bad it's a common problem

is oil filter clogged???

If its a v8 then its probably a plugged oil sump and now a bad pump. install test gauge to determine if pump is actually bad.