loss of oil at oil change

hey could anybody help? when i go to change my oil at 3000 miles i usually put my oil back in the bottles that i used to fill it, i put in 4 quarts but i return only about only about 3 quarts sometimes slightly less. i don´t see anything funny coming out my exaust and there isn´t a drop of oil on that engine anywhere. does this sound like a slightly worn valve seal, that maybe i´m burning it up alittle. i haven´t done a compression test on it yet, which would probably let me know what the problem is, and what cylinder. i notice when i take out my number 4 plug it has oil on the threads and its a bit oily looking not bad though but enough to tell. car has 78,ooo miles on it and its a 94 gt. just wandering if it sounds like what i think it is and how bad could it get and is it safe to use an oil additive to fix the bad seal. i have seen products at the auto store that say they stop leaky seals but i used to use them in my old exp and it turned out it clogged up my oil drain holes and caused even more damage. engine has been taken care of as far as oil changes but its been run hard. (you guys know what i mean if you got a honda reving next to you ya gotta give it all its got)lol.