How were your thermocouples calibrated?
The co I work for manufactures instrumentation for the oil and gas industry. We make pressure sensors, temp sensors and flow meters. Customers demand calibration traceability to known standards. Last year we spent over $100k getting ONE of our flow meters calibrated at a testing lab in Colorado. Now that it is calibrated and traceable, we can now use that as a standard reference for future manufactured flow meters.
We are also an ISO certified manufacturer. One step to achieving that is that all of your measuring tools used in manufacturing must be calibrated bi-annually. Last month the bill was $7k.
If your thermocouples were not calibrated, the error could have been due to them. Without a calibration certificate, you are just guessing.
Your calibrations showed that the CHT gauges were off. If it is true, it does not necessarily mean that oil temp guages are also off.