Thermostats are a matter of personal preference, if you feel you need one then by all means get one. We've run oil coolers for years, the first ones we used were off of imported cars as nobody had them for bikes yet. Up here in the northeast we just have an upholstery shop make a neat looking cover and put that on if it gets below freezing, otherwise we leave it open. Most dino oils have a wide operating temperature range and synthetics are even wider. The V-Twin engine is air cooled, but the oil helps cooling also. Sometimes when thing get repeated enough they become fact with out any credible research attached to it. While the years we've run coolers may or may not count as scientific, we feel it is worthy of consideration. Others mey disagree and that does not make them wrong, it is just a different opinion. Do your research and go from there. Judging from your location, I'd say your riding conditions are probably close to ours.
In closing I would remind you to be sure all hoses are protected from rubbung and have the gentlest bends possible so that flow is not comprimised. Hope this helps