Interesting from an "old coot":
When I was a kid (many long years ago) we had a "circulating pot heater" in the house. This required #1 fuel oil and that stuff was definitely not kerosene. When we finally got central heat the boiler (with a pressure-atomized "gun" burner) used #2 fuel oil.
After I went into power plant operation work I found out that there were six grades of fuel oil, #1 through #6 although there were no refineries that actually produced #3 fuel oil. Numbers 4, 5 and 6 were also known as Bunker A, Bunker B and Bunker C oil.
Of course THAT was way too simple and what was available on the West Coast was designated PS 300, PS 400 and PS something or other. The PS stood for Pacific Standard but I was never able to find such a reference in any textbook.
PS #300 was a fairly light residual oil that would burn in rotary cup burners with minimal to no preheating. It required no preheating for transport. It still was a "residual" oil that came from the refinery after the gasoline, kerosene, and lighter fuel oils had been distilled off. It was black as tar but never got that sticky unless you put it in a freezer. In the winter it was sometimes blended with a PS 200 oil to keep it from congealing in really cold weather.
Everything "heavier" than PS 300 was generically known as Bunker C regardless of what its actual specifications might be. I've burnt "Bunker C" that was as light as PS 300 and some that needed to be heated to more than 100 degrees F just to pump. Normal burning temperatures (the temperature of the oil just before it ignites in the furnace) would run from a low of about 150 to as high as 190. I've read of some Bunker C that required a burning temperature as high as 250 degrees.
Most of the oil I have burnt during my lifetime (and it has been many thousands of gallons) was burnt in steam atomizing burners. My last boiler plant burnt #2 dyed ultra low sulfur oil with steam atomization. The burner was about six feet long and weighed in the neighborhood of 75 pounds. That is a fair amount of burner when it is inserted into the boiler at eye level.
