The most beuatiful thing is that fuel oil and diesel fuel are the same thing.
.....except that one has been taxed for highway use....
But you are going the other way (highway use for non-highway purposes), so you're okay.
The other day, I was toying with the idea of buying some fuel oil and putting it in my diesel tank, but decided that with my luck I would be the one motorist who gets stopped for a fuel inspection. I imagine that one could buy a lot of diesel with the money that a fine would cost!