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a new fuel filter would be better than an old one, so doubt that's a problem (isn't your car only 1 yr old - I wouldn't have thought this would be changed anyway).
New oil of the same spec will be slightly thicker as the oil is physically chopped up ona molecular level, and the viscosity improvers in the additive pack are used up. I'd guess that this may account for 2-3% increased fuel use (equivalent from going from a 30-weight oil to a 40-weight for example), so would expect at worst 1-2mpg fall from a 50-60mpg car.
If you've noticed a bigger drop, do the usual tank-to-tank calculations and see if it is a big drop. Remember there are loads of confounding factors, such as tyre pressures, atmospheric conditions, driving conditions, etc.
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