Blowing oil from top of engine

The sending unit should be near the distributor on top of the engine.

The oil sneding unit is back there on top of the engine. Some times when they go bad they will leak oil threw the top/plug. This is the problem. It is about 3" long Sit in the middle of the engine all the way at the back.

If its reading low oil pressure I would first change your oil and filter.A clogged oil filter will cause your oil pressure to drop.Also remove your pressure sending unit and hook up a direct oil pressure guage to see what your actual pressure is.Its very possible that your oil pump could be bad.Those would be the most probable reasons for low oil pressure.The engine shut down means that your your engine has a fail safe to protect your motor from blowing up.

Are we talking out back of engine near oil pan? If so rear main oil seal possibly if we are talking on top near valve covers breather hose or pcv valve not venting motor or just too much blow by due to worn engine.

Remove the top hose, unbolt the thermostat housing, remove old themostat and install new unit with new gasket. Replace top hose and fill with coolant. Be sure to re-check the coolant level after start up, beore you drive it anywhere, to eliminate any air pockets in the coolant.

Check the pipe from the top of the engine to the intake it may be clogged up. If the engine is low in power or it is burning oil then it could de broken or worn out pistons, cylinder's or piston ring's.

Usually near the thermostat.
By the way, remove the thermostat and drive around with the car. If it still overheats, might have some other problem, like you said, temp sensor, or some kind of electrical problem.

You oil pressure sending unit has gone bad, it's common to go bad and some even leak oil. 
The oil pressure sensor is on the back of the block on the passenger side, you can get to it from the top and not from the bottom. There are (2) switches back there that screw in to the block and you can only see it from the top of the engine. The Oil Pressure Sending Unit is the one wire harness with a green and white strip on it. The other switch has more wires to it. 
When you pick up the new sensor, make sure you get the right one. make sure you get the Oil Pressure Sending Unit which sends variable oil pressure and has only one quick connect for the wire connector. The oil pressure switch is the one that only activates the oil light on older engine. 
Once you get the right part at Auto Zone for around $9.00, disconnect the wire clip with the green with white strip on it. and use a deep spark plug socket with a extension and u joint to get the sensor out. The sensor comes with teflon tape already and no oil should leak out from the block. It as simple as replacing a bolt and reconnecting the wire connector. 
Good luck and hope this helps, take no more then 10 min to do but about 5 hours for the engine to cool off LOL

Several things can cause this overheating. Have you recently done a:
1. Check Hoses for Leaks
2. Check Radiator for Leaks --- Also, Change Radiator Cap ( can loose pressure there if
cap is bad)
3. Check Water Pump for Leaks. If Water Pump is bad, you will hear a loud noise from it
4 Complete Tune Up? Should be done at least 1x a Yr. Depending on how much you drive the Vehicle. Includes: Plugs, Plug Wires, Gas Filter, Air Breather Filter, PCV Valve, Adjust Timing,
Check Belts -- see if worn
5. Changed your Thermostat & Gasket and have the correct degree Thermostat in the Vehicle?
6.. Had your Radiator Rod and Cleaned out?
7. Temperture Sending Unit (located near Thermostat) could be bad
8. 02-Sensor (Generally located in the Exhaust Manifold - needs a socket with a space in it
to allow the Socket to fit over this top wire of the 0-2 Sensor )
9. Change oil and oil Filter.
10. Change Gas Tank Cap ---

Do all this yearly ( except the Radiator Rod & Clean ( do every 2-3 yrs), and you should not have
any over-heating nor stalling out, etc. and should pass emissions as well.

With the tranny flush, yes. It's possible that they may have knocked the connection loose or broke it at the sending unit. The unit is usually located on the side near the bottom ot the tranny. With the oil change I'm going to say no. The oil sending unit is generally located on the driver side, center of the engine block near or under the exhaust manifold, or on the front of the block towards the top. Unless they moved them on me, it shouldn't be anywhere close to the drain plug on the oil pan, and there no way that they should get near the oil sending unit when changing the oil, unless the sending unit is near the oil filter, and GMC generally doesn't put the two near each other for this reason. Just thought of something. Are the gages individually replaceable, or are they all built into one display unit?