Am I the only one who used to read The Beano? You are definitely right Dan, in that it was regularly advised in owner manuals for diesel cars to add a proportion of paraffin to diesel fuel in the winter. But, much to my own amazement, a Diesel Car reader recently quoted from an old Volkswagen (I think) manual that did advise adding petrol to stop the fuel lines gumming up! No problem with the petrol, tax-wise, but, as you say, the paraffin would have been untaxed. Was that why pink paraffin was sold back in those days, I am now wondering?
I can also tell you that truckers used to light fires under their engines in cold weather, to stop the fuel lines clogging up, back in the “good old days” as my dad used to call them! What clogs things up is the paraffin wax, at what’s called the plug point temperature, and adding thinner paraffin liquid, or kerosene, helps lower this temperature when the fuel gelling can start. As you probably know, the problem is generally avoided these days by introducing winter grade diesel, and often by using additives that help stop the wax from crystallising out. But in cold winds, which rapidly speed up the cooling process, and low temperatures, the problem can still occur, particularly when vehicles are switched off and parked up overnight.