What could cause oil pressure to fail? A rattling sound appears. Oil stick shows full of oil, but oil pressure gage dropped all the way down.

Oil light is rather important it goes out if oil pressure is good comes on if oil pressure is low when it comes on damage is expected with continued operation like wearing of main bearings and pistons and cylinders can lead to a engine rebuild

no oil in sump
wire fell off oil sender unit

Check your oil level and antifreeze level or change your oil if the problem continues

Sounds like a failing engine.
Assuming the engine is full of oil, could be a weak oil pump or worn bearings.

The oil pressure gauge tells you how much oil pressure the engine has, not how much oil is in the sump.
Sounds like the new oil pressure sensor is the wrong one, or there is a problem with the gauge on the dash. There are two different sensors, one for a gauge and one for a warning light.

Could be a leaking head gasket or intake gasket.
The gunk is water or coolant. Water if the engine never warms up, coolant if there is a leak.

If the engine is not knocking or rattling when this is happening, you most likely have a bad oil pressure sending unit.

The oil pressure in an engine will naturally drop off as the engine warms up. If the oil pressure is dropping too much it could be cause by several things. I don't know how many miles your engine has on it but the oil pump may be getting weak and may need to be replaced. Worn out cam bearings or crank bearings can also cause low oil pressure. Sounds like your engine may be ready for a rebuild

You need to replace the oil pressure sending unit, this is what tells your gauge what the pressure is. For future information if your oil pressure drops again and its a real failed oil pump within a few minutes you will internally damage the engine. I would get a new Oil Pressure Sending Unit. Let me know if you have any questions.... Thanks!

I hate be the bearer of bad news but it sounds like you bottom end bearings are getting worn out. You have excessive bearing clearances causing your drop in oil pressure.