Extreme oil pressure flux!! Help!!

[MROY16]

I have a 1998 GMC K1500 Z71 offroad with the Vortec 5.7L 350ci V8.

Cold air intake
True dual exhaust (no cats, dual in/single out Magnaflow muffler)

Nothing else done to the engine this far.

Idle is 600RPM, my oil pressure fluxuates from nearly 0 at idle and back to normal at WOT. Not sure why this is happening, started happening more after the exhaust was done (cats removed). Never burned oil until today (April 12, 2014). Started burning oil for about 5mins, then stopped and hasn't since. NEED HELP!!! This is my only vehicle and I am going to university right now so don't have enough money or time to fix major repairs. I'm hoping it is something cheap and simple! CEL/MIL is on for O2 Sensor since cats were removed.

PLEASE POST ANY SUGGESTIONS AND/OR EXPERIENCES OF THIS PROBLEM!

 

[98trck]

what type oil you using, conventional or synthetic that will make a difference in the oil pressure fluctuations
'98 so it has a lot of miles on the engine so there is more blow by that will dilute conventional oil
how many miles on the oil in the engine right now?
PETE

 

[MROY16]

Engine has 102,500Miles on it today.

Current oil is Mobil1 Full synthetic. 2500Miles on this oil, usually change oil every 5000kilometers (3000miles). Next oil change I'm switching to Royal Purple performance synthetic for cost effectiveness ($10 cheaper for 6 single quarts of RP compared to 1 5Qt jug of Mobil1). I never use conventional due to the limited benefits and lack of excess lubricants.

Just replaced oil pressure sender and the gauge no longer goes to 0 at idle, but it still drops dangerously low for what I'm used to.