Crud in shitey fuel can build up extremely quickly - you would be amazed at the crap I have seen in engines from very low mileage and why I only use Shell and Chevron.....they over treat their detergents and its worth the extra p or two at the pump and, by the way, it refelcts in the mileage. In the US, they are prepared to advertise on this now, so far in the EU, not yet. It's only the lawyers putting the threats and skids on - in the US the claim is 5% fuel economy saving and, believe me, reality is a number far in excess of that.