When i can get E5, i use it because i get less mpg on E10. If i work it out on pence per mile, i find that it works out about the same & sometimes i'm slightly better off using E5. The main reason for this is that E10 produces less energy so reqiures more to give the same bang ? Also, due to the higher ethanol content of E10, it's more likely to absorb more water & cause issues. Unfortunately, i'm finding it more & more difficult to just drive on to a local forecourt & find the one or two pumps that may have E5 ? The only good thing i can say about E10 is that it does appear to have cleaned my fuel system, & probably the catalytic convertor as well, so when i went for the last MOT, the emissions were lower than last year, even though i had put E5 in prior to the MOT ?