The exact difference is not known to me, but here is the limited information I do know: Home heating oil and diesel used "on-road" have vastly differing amounts of sulfer content. The diesel is also dyed so inspectors can tell the two types appart (Red for road use, green for non-highway). The number relating to diesel is related to it's flow characteristics. For example #8 "bunker oil" needs to be preheated just to flow into burners in large industrial boilers. I don't know if this helps, good luck.