Well technically the engine would run at lower oil temp with lower viscosity oil. However head temp is primarily influenced by coolant temp.
I am a fan of HDEO in many American OHV engines. However I dont know enough about the Cologne V6 to make a recommendation.
If these engines prefer 10W-40 over 10W-30 a HDEO like Mobil Delvac 1300 Super, Chevron Delo 400, or Rotella T 15W-40 might be a good choice. They tend to run to the heavier end of the spectrum close to a 50 weight.
If you can find it all of these HDEO also come in 10W-30 but it is pretty hard to find. These also tend to run to the heavier end of the spectrum.
Gene