Ok, here's my take on it.
Throughout the life of my crdi i30 I have tended towards using premium fuels wherever possible.
However, having said that, the last few tank fulls have all been on "cheap" Peak diesel and for the last eight tanks I have got over 1,000 kms per tank easily.
I do feel that it doesn't run quite as smoothly and quietly and that there is more diesel clatter, but this is purely subjective and I don't have anything to back it up.
One thing I have noticed during my i30 ownership though, and maybe someone can explain this to me, is that once I have a tank of fuel that gives me "only" 850 kms, the following tanks also only give me that range.
But then if I go on a long highway drive and attain 1,000 + kms on a tank of fuel, all the tankfulls following give me in excess of 1,000 kms also.
Does this have something to do with the ECU resetting itself somehow?
I have kept records of every tank of fuel ever bought from day one with the type of fuel, kms travelled, etc and I can show this time and time again.
At the moment I am in a run of over 1,000 km tanks and as I said, on cheap fuel.
A friend of mine about two and a half years ago, bought a new Kia Sorento Crdi and when it was delivered the dealer gave this piece of advice:
Only use quality diesel, eg BP, Caltex, Shell.
If you have any warranty claims regarding the fuel system, the first thing Kia will do is take a sample of fuel and analyse it and if it is cheap diesel they will not honour the claim.
I fail to see how they can do this when the cheap brand fuels are readily available and they are selling their cars in this market, but I'm just repeating what he said.