heard the sediment theory too, tbh heard it sits at the bottem so when fuel runs low thats when its most likily to mix with it as it sits at the bottem.
however in my experience with general cars never had the issue and i run them in the reserve regularly on occasions, so wouldnt think the new m5 would be any different.
as for the cold and running it in the reserve, it did cause a slight breathing issue on my e46, as air/condensation got into the fuel tank when it snowed here for a week and temp drop.
my advice.. its a m5, fill it up so you can keep driving it !
