Hi
I have noticed quite a difference in price per litre between E5 and E10 fuel. Perhaps up to 8 cents per litre difference.
I have heard that with E10 fuel the car will use slightly more fuel and will offset the saving somewhat.
As far as I am aware most petrol cars built in last 10 years will run perfectly happily on E10.
Any observations?
biofuels might do more ecological harm than burning dinosaurs
high performance cars might want a higher octane rating
Other than that, as you say, most modern cars will run on either.
I have a 2005 Nissan Elgrand 3.5L which has used E10 since I imported it and has never had any problems with it, neither has my Peugeot 5008 which has done over 100k kilometers on E10.
With the Elgrand the slight extra consumption definitely isn’t noticeable 
I’ve also heard that E fuels are somewhat hydroscopic, so if it’s a car that’s only occasionally used then it shouldn’t be left full.
John - my understanding (from aircraft and car mechanics) was that if a fuel is more hygroscopic then the tank should be left as full as possible to minimise water uptake. I’m very happy to be corrected though! 
So I believe - it’s the ethanol that is hydroscopic and (completely anecdotally) I have heard that it is possible for E10 to absorb enough water to actually separate into a gasoline phase and a water+ethanol phase. In fact if you want to remove ethanol from E10 (or E5) mixing it with water as a differential solvent should work very well.
As for full tank vs empty tank - I don’t know. Having literally no fuel in the tank would prevent you getting an ethanol/water mix at the bottom and filling the tank completely would elliminate any (moisture containing) air pocket above the fuel.
However I suspect hat it is not possible to fill a car petrol tank to precisely 100% full, no air and even if you could there are breather systems to allow air back in as the fuel is consumed so I don’t think you can prevent all contact with air/moisture.
I wonder how long it takes to happen, I have left the Elgrand with a full tank for 4 months on a trickle charge and haven’t had any problems over the years.
Officially you aren’t supposed to leave gasoline more than 3 months - even without worrying about the ethanol content it starts to self polymerise and can gum up fuel lines and carburateurs.
Carburetors, that’s a term I haven’t heard used with cars in a while 
OK, injectors then 
I suspect the mental association was because one of the jobs on the “to do” list is to get the tondeuse in action again (last used pre-covid) and it has a carburettor.
Not sure why I didn’t spot that I had spelt in in French though.
Showing you age there 
You could well be right. There may have been a bit of the fuel “going off” in the advice I got too. Regardless, I haven’t paid any attention to it, as usual 
Some of the problems with E10 are long time degredation of parts rather than something quick. I run E10 through France as I burn through it rather quickly so its not sitting for long. Gets a treat of E5 when back in the UK and sitting for longer. Engine feels smoother with E5 and gives a little more mpg but not really enough to justify the extra.
Both my Hondas (2009 Accord Estate and 2014 NC750S motorbike) are designed for and are happy with E10. I have tried E5 in both but didn’t notice any difference in performance or economy, just the difference in price, so I went back to E10. And that’s mostly supermarket E10 as well.
I do fill up with brand-name E10 about once every six months (or if I need to fill up and am not near a supermarket or supermarché) but that’s more based on superstition than logic I think. 
Yes, my pal was telling me that he’d a motorbike, a KTM I think, which had a small pump in the petrol tank (
) which went rusty and had to be replaced, a fiddly job it would seem.
As an aside, he’s just bought one of these, it’ll be delivered any day now.
There are less bit’s on it so it should be better built, one of my neighbours has one and on close inspection it looks like it has been built out of rejected airfix parts.
Nothing wrong with Airfix, once you had the right glue. Probably the same for the new Ami. Though he says trying to actually get delivery is a nightmare. I don’t know if it’s Darty or Citroën or what. But the good news is you can “tune” them. I think you can get them up to 65kph.
I should add that they also seem very expensive to insure. I suspect the insurers think they are either for 14 year olds of banned adults and damn easy to steal to boot.
Absolutely, some of my friends worked for Airfix 
You will note I said rejected airfix parts
It rides badly, it handles badly it has terrible build quality, it is uncomfortable and it rattle’s like an empty Guinness can with a widget, can you tell if I like them yet.
Are we talking about the Ami or MG now