The price differential change came about at the same time as the major switch to USLD. I'm sure that the hydrotreatment to remove most of the remaining sulphur is the primary excuse for higher diesel pricing.
I once believed that might be true. But the spread between #2 fuel oil (500+ ppm of sulfur) and 15-ppm-or-less ULSD is only 4 cents, according to the Cash Prices page of the WSJ. And it has been in the range of 4-9 cents a gallon for many years, basically from the time of the switch to ULSD back in 2005 or 2006. I'm guessing that if there was ever a shortage of ULSD then the price spread would increase; but most refineries in North America, Europe, and the Caribbean only produce ULSD. And the only time they sell something by another name is when they get a bad or contaminated batch, that might exceed the 15-ppm limit.
Just because two things happen at about the same time, does not mean that they are related, or that one caused the other.
That is especially the case when there are other very plausible explanations. I am pretty sure the reason for the widened spread between gasoline and diesel prices is due to a shrinking demand for gasoline (more fuel-efficient cars, ethanol replacing gasoline in the blend) and steady-to-higher global demand for diesel and other distillate fuels.
When refineries crank up production to meet the demands for distillates, they produce excess gasoline that gets dumped on the market at whatever price they can get. That's certainly what happened in the US over the last several months, until about 5-6 weeks ago when maintenance shutdowns at a number of refineries reduced the surplus production of gasoline and shored up its price.
Additionally, when refineries end up losing money on the sale of cheap gasoline, they tend to reduce refinery runs and this cuts into the supply of diesel and other distillates. So, cheap gasoline is sometimes the
cause of expensive diesel.
