Had my Stilo Jtd over three and a half years now, usually bought my diesel from a local garage that just bought fuel on the local market, car always did around 48-49 to gallon, a lot of local trips/cold starts during week, after that garage closed down started using WCF (west cumberland farmers!) diesel from another local garage - instantly doing 50 - 51+ to gallon, more like Fiats claim, although a couple of pence dearer per litre. As that was getting more expensive I started using BP Regular diesel and straight awat saw consumption drop to about 45 - 46, this week including an 80 mile shopping trip its averaged 43.8 over the last 240 miles - SHOCKING!
I know its been a bit colder recently but surely it can't make that kind of difference, colleague at work said her BMW 320d was doing about 5mpg less on BP and switched back to WCF, thats were i'm going next time, anyone noticed varying consumption with different brands

WCF 110.9 a litre = 10.06p a mile BP 107.9 a litre = 10.88p a mile

Not quite the same but I have just given up on Shell V power after a 3 month test and have reverted to Tescos finest (plus Millers Diesel Sport 4) as, with V power, my MPGs dropped to the mid-upper 40's.

With bog standard diesel I'm again getting around 51mpg......

The only thing noticeable with V power was, perhaps, a smidgen less smoke under hard acceleration

I stopped using BP fuel as their pricing (in Aberdeen at least) is almost 10% above the others who price match to Asda