I started mixing E5 with E10 in varying ratio’s since the cold weather we had, mainly because when pulling off at junctions the engine would want to die on me as if pulling off in 2nd or 3rd gear (mine’s a manual box). The car reponded better pulling away, but, the extra 1-2mpg wasn’t really woth the extra outlay.
I’m sure I read that the engine management computer alters various parameters dependant on the fuel, but to gain the extra performance of higher octane fuels you need to run a few tanks through (i.e. it takes the computer a while to adjust to different octane fuels).
Having said that, I did 3 full tanks of premium/super unleaded in a row and it didn’t seem to improve the mpg.
My ppf warning came on whilst using E10 and I did revert to E5 whilst trying to unclog it, but I can’t say whether the E10/E5 played a role in the ppf issues.