2005 chevy silverado 5.3 Liter 1500 4x4 , im havn a problem with the oil pressure gauge fluctuating from 80 to zero sporaticly. Starts out showing pegged at 80 psi, then will occasionally drop to 0, then bounce around. After 4 or 5 minutes it will peg at 80 and stay there the rest of the time vehicle is running.

Knowing how these gauges work , I'd say you have an instrument cluster problem not oil sender unit. 2003 - 2006 GM had bad stepper motors in the instrument cluster . If there was a problem with the oil sender the gauge would default to zero .
Engine Oil Pressure Gage
The instrument panel cluster (IPC) displays the engine oil pressure as determined by the powertrain control module (PCM). The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The engine oil pressure gage defaults to 0 kPa (0 psi) or below if:
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The PCM detects a malfunction in the engine oil pressure sensor circuit.

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The IPC detects a loss of class 2 communications with the PCM.

If you don't know what class 2 serial data communications are ,google it. How to repair GM Instrument Cluster Speedometer Gauge Stepper

Either you have way to much oil in the truck or the pressure switch is no good.

that is a sign of the wrong oil grade. that truck takes 5 w 20 oil

is this a problem if oil pressure is 80 when i start it up?

More then likely the sending unit is faulty, but to be safe hook up a mechanical test gage at the oil pressure sender port and check oil pressure hot engine, if it is steady then replace the sending unit.

i want you to do this , UN connect the oil pressure switch and let me know if this way can fix your gauge problem .

check the filter,, if its bad enough your oil won't pass through it, making it look like your oil is full, but its not moving, Chevy's have whats known as a stepper motor also on the oil gauge,, whats bad about that is if thats gone bad, you have to replace the whole gauge cluster which last time I checked is 250.00

It could be the sending unit. If the truck is low on oil it will cause something similar. Be sure the oil level is topped up. The only way to be sure it so measure the actual oil pressure at the engine with a proper gauge..

what is the normal reading?
it may have to do with the viscosity of the oil if it is thicker the pressue will show higher if thinner then it will show lower.

if it is showing extreamly high and does not drop once the engine has warmed up then your oil or sump filters maybe blocked.

I hope this was useful.

is your oil level low? make sure it has enough. if that is ok maybe the sender or wire to it is bad