In the past, a restrictive air filter or air intake, could have resulted in the engine running "too rich" which had a negative impact on fuel economy. However, since modern fuel injection, and computer controlled engines have been developed, this is no longer the case.
Here is a very general overview of how modern engines deal with air flow:
-The computer measures the amount of air entering the engine, and the temperature of that air, and then predicts the precise amount of fuel needed to react with the oxygen in the air to make the most fuel efficient burn.
-Then as the exhausted gases are passing out of the engine they are measured again (sometimes twice) to determine if the fuel is all being burnt, or not
-Then the computer makes fine "trim" adjustments to the amount of fuel being injected until it is satisfied that all the fuel is being burnt, (also, the least amount of NOx and SOx are being produced)
As you put on a more restrictive, or less restrictive air filter or intake, nothing happens to your fuel economy, the computer accounts for the amount of air entering the engine regardless of any restrictions. HOWEVER, you can drastically reduce the amount of POWER the engine will produce by restricting the air intake, This happens because the computer cuts back the amount of fuel injected as the air is cut back. If you have a 200hp v8, but only allow enough oxygen into the engine to produce 100hp, then the computer will only inject enough fuel to make 100hp.
This is all assuming the engine computer is working properly, and there are no "check engine" lights, and the engine has warmed up already.
Unfortunately, there are those who don't believe this (or understand closed loop operation) and will still spend significant $ to buy "low" restriction filters (oiled or not) for their vehicles believing FE will increase.
Also, to be clear, the possible loss of peak power due to intake restriction is hypothetical for virtually modern cars since the air box, intake, and filter are not restrictive even at peak power (but for a clogged filter). There have been plenty of dynos run using a low restriction air filter vs an OEM air filter vs different intakes, air box and so forth on a modern stock car and there is virtually no perceivable performance benefit from modding the intake but for "sound" - some like to hear that engine / turbo on hard acceleration.
The OEM filter and air box are designed appropriately and there is no benefit to modify intakes or filters on a stock car. If one is worried about air flow restriction from a dirty filter buy a filter-minder like this one and install on your air box. One will likely get a better return from a filter-minder than a K&N filter as the air filter change interval of 30 k miles on the Ford C-Max is likely conservative.