Gonna do a long term experiment on mobile1 15w50. Temp range where I live goes from -10f to 100f. Gonna use 2 vehicles. A 2001 k1500 4.8l automatic. 205k on it. No cats. No egr. No knock detector. Known cam lobe wear. I forget which one but it was for cyl 7 or 8. Absolute beater of a truck. Was running t6 5w40. 1st impressions of the 15w50 is engine is quieter. But feels more sluggish. According to the horrible gm guages oil pressure has risen about 5 to 10 psi. Idles at 40 to 50. 60+ running down the road. Also about a 5deg rise in engine temp. Running about 180 on a 160 thermostat. 2nd vehicle is a 2008 Toyota yaris 1.5 automatic. 195k miles no catalytics. Was the wife's car and has been abused. Haven't driven that yet since the change from t6 5w40. Wife currently drives a 2011 nissan cube. But I'm not brave enough to switch that from t6 to 15w50 with it being our nice car and with winter coming. Learned alot on bitog and enjoy reading people's posts
Maybe back in the day thicker oils were better to a certain extent, but you said the engine feels sluggish and there is a rise in temp. Your engine is working harder to push around thicker oil so that is why you are seeing the rise in engine temps.
Back in the 80;s I was running 20W-50 during the summer and the coolant temp gauge was pegged at 220 with a mechanical fan. I then went to 10W-30 and the temp gauge went back down to about 195 or so. Was my temp gauge accurate, who knows, but I wish I had an infrared gun back then to take some temp readings.
Good Luck