Mpg went down after air filter change and why that may be a good thing

I acquired My 2012 Prius three weeks ago. It had 218k miles and the ice was just replaced with a 30k mile ice from a donor car. The car is amazing and in every way drives like a new car. I’ve put over 4K miles since I’ve owned it with the only quirk being the infrequent prius cough particularly in cold damp weather. It has not used a discernible amount of oil in that time. I’ve just replaced the air filter that was quite dirty and appeared very aged with a fram and my mileage in early driving appears to have dropped from mid 50s to high 40s. I’m ok with that because I saw the original engine that came out of it melted a hole clean through the pistons. So my supposition is that somehow the dirty filter creates a lean condition that results in improved mpg yet higher engine degradation from higher cylinder temps and less lubrication for the rings. Thanks for any feedback.

 

Idk I was shown the engine outside the car with a hole in the block about the size of a golf ball. Thanks for the suggestions, I’m gonna leave it alone as long as I’m getting factory numbers on mpg. I’m not convinced the hypothetical lean condition from a dirty oil filter killed the engine, or the previous owner was not monitoring oil level and burned out that way, it’s all speculation. My personal interest is not in max mpg but rather max engine longevity, btw the cough I described, basically ice startup stutter, likely will not present again. To early to tell though

 

I agree Logically the dirty air filter should have created a more rich mixture and therefore higher fuel consumption per mile , that’s the mystery because mpg went down, How could a more restricted air flow increase mpg? My interest is engine longevity, hypothetically does a high mpg engine wear out faster than a lower mpg engine?