On my last few flights in the Bonanza, during my instrument scan, I noticed that the oil temp started fluctuating slowly, up or down by 15*C. It never reached red line, and the oil pressure stayed very stable at 40lbs. It appeared to be an instrumentation problem rather than an engine problem.
On a Continental IO-550, the normal operating temperature is 80*C and redline is 116*.
At first, I thought it was the vernitherm on the oil cooler unit that was stuck, but a call to the America Bonanza Society put me in touch with a tech that quickly diagnosed the problem over the phone.
It turns out it was a defective oil temperature probe. This probe screws into the bottom of the oil coolers on IO-550 Continental engines.
A newly installed probe runs about $225 and it now works perfectly. The indicated temperature is lower and there is no fluctuation in the gauge in cruise flight. We accomplished this switch during a regular oil change so there was no additional down time.