turbo or valve seals? blowing smoke

[bbullsj]

i have an 89 supra turbo automatic. once the car is warmed up, most anytime you let off the gas and let the engine braking slow the car down some there will be smoke coming from behind the car. if you get back on the gas some it will go away. i was wondering if anyone could tell me if its more likely my valve seals or the turbo. everything is completely stock on the car and it has 170,000 miles on it. im pretty sure the head has never been off but i cant say for the turbo. it probably hasnt eaiter but i dont know for sure. i added a quart of trans fluid in with the last 2 oil changes(4 quarts of oil and 1 quart of trans fluid) thinking that might tell me if the valve seals were bad but it hasnt helped. or the seals could be too far gone for it to have made any difference. if anyone has any ideas or things i could check to narrow it down more that would be great.