[chittychittybangbang]

2nd check the engine oil and post what part number they used for the oil.

Thicker engine oil will lower fuel economy.

Winter fuel is cut to lower the gel point and will also lower fuel economy. This is the only thing I can think of.

There is no obvious reason why your fuel economy is going down. Your fuel economy is already at the upper end of the bell curve so I don't think it's your driving style or any quirk of that particular car and I would expect it to stay there.

 

[chittychittybangbang]

Coolant won't make any difference as long as it's pink. Only use G12 in the coolant system.

 

[chittychittybangbang]

Strange. Maybe it was just that tank of fuel.