The car is a 2008 Subaru Impreza 2.5i with 22 k. The oil is mobil - non synth.Most manufactures would say that anything under 1 quart per 1000 miles would not cause mechanical issue. Not that I actually agree with that, but if you're using 1 quart per 5000 miles, not likely to get help from Subaru. Or Ford, Toyota, Honda, etc.
+1Maybe a comparison of the 4 spark plugs would show a cylinder that is 'different'? Maybe that's where the bad ring or valve seal is.
Also, change/clean the PCV valve/hose.