when i was in school a few years ago, i drove a tank truck delivering home heating oil in the winter in pa., the truck carried both #1 and #2 heating oil, #2 was diesel and #1 has kerosene, at the refinary we loaded from diesel / #2 and the kerosens fill stations. it always has been #1 and #2 fuel oil, the first diesel engines was designed to run about #4 fuel oil but #2 was already in use as a fuel oil for heating and other uses, so #2 started to be used, after awhile #2 started to be called DIESEL fuel