I can't speak to the point about different formulations for hot or cold, as my only direct source of knowledge was completely unaware of it. A dimly recalled web search from years ago was also fruitless, though the mechanic for my BMW bike in the U.S. always maintained that Euopean gasoline and motor oil was refined and formulated to a much higher standard than common in the U.S.

However, as for the idea of costs being buried under taxes - absolutely no way. Prices here can shift 5 euro cents a liter in a day (or something like 25 US cents a gallon) - those shifts have absolutely nothing to do with taxes. However, they do seem fairly directly tied to the euro/dollar exchange rate, and the price of crude. Not perfectly, and as always, the price goes up like a rocket, and falls like a feather, but the correlation is pretty clear, as is the causation.