Attached is a snip from the wiring diagram for the 1973 1800 es. 66 is the oil temperature gauge and 67 is the oil temperature sender. The oil temperature gauge is supplied via the voltage stabilizer which also supplies the temperature gauge 29 and the fuel gauge (out of the picture). If the fuel gauge and coolant temperature gauge also behave erratically, I vote for a problem with your voltage stabilizer. On my 142 the voltage stabilizer would occasionally have fits causing the temperature gauge to pin in the red and the fuel level increase.
If the other gauges behave normally then the problem is likely
-in the gauge,
-in the gauge wiring, or
- in the sender
Refer to the service manual for the gauge test. The wiring diagram says that the sender wire is brown so you may have the incorrect sender. However, brown covered with oil and dirt can look pretty black. Check the service manual or get advice from somebody else on the correct location for the oil temperature sender.
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