I'm mostly on a Ford Ranger board. Mine is newer but we have many members with older trucks and Bronco II's. The 2.9's seem to run forever IF the heads don't crack from high heat. Could I recommend to those folks to use 15w-40 diesel oil? Thanks.
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.