Posted this to the oil helpful hints thread and have gotten no response, thought I'd try it here for greater visibility. Read through all 12 pages here. My shop manual for a 2008 Road King says if adding oil that is NOT Harley's super duper stuff, it should be diesel oil- I quote the service manual: "If it is necessary to add oil and Harley-Davidson oil is not available, use an oil certified for diesel engines. Acceptable diesel engine oil designations include: CF-4, CG-4 and CI-4." Then it goes into viscosities. I have read the labels on most of the engine oils discussed here and have found no diesel specs listed. I originally used Mobil 1 15-50 automotive oil and was quite happy until I found the diesel requirement. There is only one oil I could find that had both the diesel spec and the viscosity range needed. Automotive oils with 20-50 viscosity are easy enough to find, diesel oils with 20-40 viscosity are easy to find, but only the 15-50 oil I'm using now has the desired viscosity range AND the diesel spec. How important is that diesel spec? I'd go back to either Mobil 1 or Mobil V twin if the diesel spec is not important. Your recommendation of Spectro 6 speed for the tranny is quite convincing. I will give it a try.