Art,
You might try one more experiment to test the accuracy of your
oil temp. Since the bourdon tube from the temperature pickup is open to the airflow beneath your car, it may be getting cooled by the airflow. Try wrapping the first few inches with some insulation...even a rubber hose taped over the tube would work.
One my friends with a small-block had this same issue, and after this "modification", the
oil temp read 10-20° higher! Having said that, you will probably find that you will need to block the
oil cooler intake in cooler weather. I like the foam idea...sure beats the cardboard I used last year.