Fuel economy is slowly decreasing in my 2006 Ford Escape Hybrid FWD. I'm at 106,000 mi currently.

I usually get a steady 31 mpg with mixed driving and I'm happy with that, although lately I've gone down to about 26 mpg.

Just a few hundred miles ago I changed my oil and oil filter for the first time on my own. I bought the right oil, 5W-20, and I went with the Castrol synthetic blend for cars with over 75k miles. I put 4.5 quarts in and the dipstick reads right. Also changed my oil filter with a Bosch and it matched up exactly with the old filter.

My spark plugs are less than 2,000 miles old (and properly gapped).

My air filter is K&N and it's clean.

My hybrid battery filter is new.

I recently purchased an OEM fuel filter but haven't had a chance to install it yet, not sure if this could be the problem.

I'm probably due for a new battery. I bought the car at 80k miles and for all I know my battery could be the original. Not sure if this could contribute to the problem.

I put regular unleaded fuel in my car just as I always have.

It seems that my fuel economy has been slowly decreasing since I changed my oil. What could I have done wrong?

 

Did you happen to use spark plugs that were not the OEM Motorcraft ? Have you checked your tires' air pressure? Many use 40psi or greater for maximum mp (and also use light low rolling resistance tires).
Winter gasoline/cold temperatures can also cause significant decreases in mpg if everything else is the same, batteries work better when they're warm.

 

I used NGK spark plugs however I've been running them for about 2,000 miles without any issues. I monitor my tire pressure closely and keep it around 38 PSI.

 

Depending on where you are winter makes a big difference. Even more so if you do a lot of shorter trips vs a few longer ones.

If you're in a colder place put in a block heater set up. It warms both the engine and traction battery. More recent years took out the wiring that went back to the traction battery. However the plug in on the traction battery for its heater element is still there and could be used with the Ford transformer setup.

 

Poor alignment and dragging brakes will hurt mileage. As will changes in driver or driving patterns.

 

Poor alignment and dragging brakes could be part of it... but they'd have to be pretty bad to affect things as much as you seem to be seeing.
Don't discount the block heater thing. I saw a big difference after I put in the block heater. Of course my winters are truly cold out here.
With regard to spark plugs I did an experiment on my Suburban (500k+) for several years and ran different plugs for about 50k miles to see if they made much difference. Result: little noticeable difference. So for plugs I'd select some good one's and be done with it. On the Escape I change out plugs every 50k and vary the type to little overall change in MPG.