Archer high oil temp

Now that summer has come to Arizona, my oil temp is running about 2/3 of the
way between 180 and red line. My mechanic checked the oil cooler bypass
valve and said it was OK, so I had decided to send in the oil cooler for
cleaning and service. Checking the engine log for the last time work was
done on the cooler, I found an entry about 5 years ago saying the cooler had
been cleaned, then the next entry said the oil temp gauge was reading about
20 degrees too high, but there is no entry for corrective action to the oil
temp guage, wiring, or sender.

Looks like I need to check the gauge system myself for accuracy and probably
replace something. Do you just drop the sender into hot water and compare
the panel guage reading with a couple of cooking thermometers? If there is
a discepancy, is it the sender that should be replaced first, then replace
the guage if necessary?
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Regards,
Mike

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