This just sounds like the same old wives' tale. Please understand that the thermostat controls the LOWEST engine operating temperature. The fan control algorithms establish the highest operating temperature. Retain you OE thermostat. Installing a colder thermostat will NOT lower engine operating temps in hot weather, especially in stop and go driving, where the engine will heat up to 100C or more.

At 100C the themoclutch on the engine driven fan "engages" and the auxillary electric fans engage high speed at either 105 or 110.

You want at least 80C to keep moisture from condensing in the crankcase; 71C is too cold. In addition to allowing more condensate, a low temperature themostat will decrease thermal efficiency by removing too much heat from the combustion process, which means higher fuel consumption for a given operating condition.

As radiators age they build up deposits that decrease their heat transfer deficiency, so it is common for a car "to seem to run hotter" as it ages. You can minimize the rate of internal radiator deposit build-up by using MB antifreeze (Zerex G-05) or a Dexcool certified blend. Using the traditional green antifreeze will build up deposits much more rapidly because of it's high concentration of salts, especially silicates. Dexcool has a pure organic corrosion inhibitor package, which has no inorganic salts whatsoever.
MB antifreeze, which is commercially available as Zerex G-05 is a "hybrid". It has an organic inhibitor package with a very small concentration of silicate.

Duke