Echoing what some are saying here, use regular good fuel from Shell/Esso/Texaco/BP/Jet. Every 5th fill up which I guess would be the same as every 2.5k miles, perhaps give it a tank of their most expensive fuel, (Vpower, Supreme, Ultimate) not sure if Texaco and Jet has an equilavent or not. This should keep things going nicely. Eco mode, can't comment, as I haven't got this function.
Start/Stop I usually keep it enabled, when I approach a roundabout or when in crawling traffic, I would turn it off so engine doesn't turn off on me. If i am crawling, and I end up just stopping at the a red light, I would turn it back on (SS), and the engine goes off. I usually let it do its thing at the red light otherwise and turn the engine off. Does it save fuel, I believe so, perhaps a couple of mpg here and there. Didn't calculate, but pre-lockdown my usual jorney of 3-4 miles would usually get me around the 30mpg mark in city driving, with medium traffic, return leg about 37mpg. This is with startstop enabled with usually 7-8 traffic lights, of which 4-5 are usually red and me waiting with engine off, but 3 of these are long waits..like 30secs. As lockdown slowly lifted I did the same said journey, of course with much lighter traffic, but the same traffic lights, and as the battery was low, no start-stop avalaible, and both jounries I got around the same mpgs. However since the traffic was lighter and shorter waits at the lights, I would say Startstop would save fuel, as long as your wait times being stationary are long 20+ secs...it's worth using it. I can't see constant on off of engine just for 3-5 secs be any good for any of the components.