Thermostat is too cold . Should be using a 180 series at a minimum. Using a 160 will just make your engine take longer to reach optimal temperature, leading to moisture in the oil that never gets burned off.
As far as using the Mobil 1 15w-50 goes as an experiment, that’s very old news. The results have been in for decades.
Hundreds if not thousands of vintage car owners have been using it for at least 20+ years, myself included, in all types of temperatures. In seldom used cars, and in daily drivers. My daily driver vintage Mustangs and Shelby’s never had issues starting or driving with the 15w-50 in temperatures ranging from zero F to 120 F. Excellent wear protection .
But I do agree with others here, that Mobil 1 0w-40 is a better all season choice. Much less drag and with all the wear protection of the 15w-50. Surprisingly, I found that both oils, when hot, gave identical oil pressure and temperature.,
Z