My only comment to an otherwise “C” level piece of writing is that if our “coustomers” (the people that ask us for advice here) used your 10% reduction in fuel mileage to indicate a mechanicaly damaged engine we would be flooded with posts asking what type of engine damage we think they have.
Your mileage can change by 10% for so many other more likely reasons than engine damage.
Dont for one second believe that an entire quart of oil is left in your engine after an, at temperature drain plug removal.6oz at most.