hmm i was having the same problem when i first got my teg. i was actually burning it, but didnt realize becuase i had a stock exhaust. as soon as i got my apexi (because it stuck out farther than the stock muffler) i noticed the whole rear of my car was getting dirty quicker than normal haha i soon realized all that "dirt" was actually oil :/
do you have the stock exhaust? are you sure your not leaking from anywhere? like the oil pan or somewhere?