SNAKE OIL IMO.
Your gauge readings may or may not be accurate. My suggested approach to temperature related issues.

Step # 1 buy a thermocouple and digital meter. Make sure to get a T/C that matches the meter as there are different types that will skew your readings. The most common are type K or J. Many of the newer DVM's (digital volt meter) have a plug in for T/C's> they can be had relatively inexpensively as can the T/C probes

Step #2 Buy a T/C probe with an .125 OD SS sheath ( enclosure) around it. Buy an extension lead that is long enough to put the meter in the car.

Step #3 Install the TC probe as close as possible to the thermostat.

Step 4# Put the meter in the car. Take passenger along to record the ACTUAL temperatures under various conditions. You may be surprised to see the things that affect the temps such as a semi in front of you, climbing a slight grade etc. Example: with a borderline radiator, Going north on I-29 has a tendency to slowly raise temps, turning around and going south lowers them. This typically indicates an undersized, partially blocked, or inefficient radiator core. Sometimes turning the heater on will help as it adds a small amount of cooling to the system.

The above efforts will confirm or invalidate your suspected problem and tell you exactly what the temps are and when.
Once you have that information , you will then know how to approach resolving the issue if there is one