Based on the Detroit Diesel bulletin, it seems that slow idle is indeed quite inefficient, and excess unburned fuel can mix with the oil, leading to the formation of sulfuric acid over time. This acid can then attack the bearings, especially if the engine is off for an extended period. I typically shut down my engine after 4 to 5 minutes if it has cooled down and the turbo has reached normal speed. Does that sound about right?
Best,
Pres