I'm in the same boat - I understand octane as an anti-knock component of petrol: my own car requires a minimum of 98 RON on its current tune, that being V-Power or Momentum only
If I ran it on My Mum's Happy Shopper fuel, the ECU would back of massively and I'd probably lose 30bhp or more as the engine tries to avoid detonation
From the little research I've done in the last couple of days, Diesels use Cetane to boost "something" that makes them betterererer fuels
BP Ultimate is literally diesel + injector cleaners + a LOT of Cetane apparently.
I have no idea what the chemistry is because it certainly isn't knock related
but cetane's where it's at apparently.
The reality, however, is that you WILL NOT GET VALUE from using premium fuels - only use it if your car NEEDS it. I don't care if you get another 10bhp or whatever but it just ain't worth it. BP Ultimate is literally 10p per litre more than BP normal! That's what, £6? (I don't know how big the F30 tank is) Well that's about a gallon of fuel. Does it really give you 50 miles extra mileage? Is 10bhp (in a diesel
!) really any difference at all? It's madness I tells ye