In reply to timjones:
Well - I don't consider myself dumb, have owned and driven countless cars over the years and its not always that obvious - especially with modern diesels - many start instantly, don't show a glow plug light unless its very cold, have a rev counter that goes up to same numbers as on a petrol (not all have one with a reduced range) and all are pretty quiet these days. And I'm also no proponent of blame culture but, if something is critical, and an argument/issue is going to come down to word of mouth and who said what, its foolish for any firm not to take sensible precautions. Like stating what type of car you have and what fuel it takes.
If you are told 'it's petrol' and there is nothing obvious to suggest otherwise, you may well not notice 'diesel' embossed in black on a black cap (or whatever) until too late as you are simply not looking for it - you 'know' its petrol because thats what they said. To protect from that, whacking great signs and something on the documents seems far more sensible than 'telling people' or not, or telling them the wrong thing possibly.
Most mistakes seem dumb in hindsight but if you are not expecting to have to think about/check something, you don't.