Well, you have been through; allot of issues here. Each aspect, of the cooling system; needs to be checked.
You definitely need to fix the fan problem and get that working correctly. If you mixed green and pink vw spec coolant; you need to check for any thick reddish junk that occurs when they mix.
That gooey stuff, tends to gather at the upper levels of the cooling system; so, i removed the hoses, coolant bottle, used dawn dish washing soap, to scrub it out, washed everything with water. Then, blew out all the hoses, flushed the system as best i could.
The oil pressure light; can be electrical, related to the coolant temp sensor or a actual low oil pressure issue. I had a similar issue: oil pressure tested in spec but found the coolant light acting weird (light stayed blue, wouldn’t”t go off, blinked blue) , had a trouble code, for the coolant temp sensor. Found terminal to the sensor, had terminals broken off; got new terminal repair wires, plug and sensor. That solved the sensor issues and the oil pressure light/beeping noise, went away, stayed away.
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