Is this an off road construction truck? What driving conditions plugged an air filter in 2000 miles?

Your engine is an air pump, and a Diesel especially a turbo diesel pumps a lot of air compared to a gas engine with its throttle plate..

The wix filter when new creates very little restriction in the intake tract. The oil change place filter may create a little less restriction at the expense of poorer filtration efficiency (and shorter engine life) but not enough to measurably affect fuel economy. The fact that it plugged in 2000 miles runs contrary to this.

Better recheck your math.