2010 1.6tdi mileage drop

Hi guys.

I bought a new 1.6TDI Octavia, with the new common rail engine, in March. It has settled down to a routine ~57 mpg over mixed but consistent motoring on Esso diesel.

Last month I noticed the adverts for the "new" shell diesel that suggests it can give you an extra 10% mpg for the same price, so I filled up with that. The MPG has dropped to 53 over a tankful.

Yesterday I filled up with their Ultimate diesel, and although the thing did seem to go better, the mpg figure for steady motorway cruising has dropped by 10% or more. I did not drive any differently.

As an engineer, I know I should not trust this one "dip", but it is noticeable. Has anyone any thoughts on Shell diesel. I really can't believe it would cause such a bit drop.

The other possibility, is that the engine management system has gone into enriched mode, to clean the dpf. I know this is a problem with Fiats, but not sure how this system is triggered in the VW group engines, and if there is a similar problem with overfuelling. Off now to check if oil level in sump has gone up, and to put this query on the Briskoda site. Anyone know of any link that would explain the dpf system in my motor before I take to the agent

Thank you