The use of synthetic MIGHT cause the car/seals to leak oil because of the different weight (I believe) It's good to convert to it and use a high performance synthetic blend, but it might cause you to leak in some places. I am currently running 5W40 Eneos oil in my 94 TT which I boost up to 20lbs and live in Nevada. I have had no problems and I recommend their oil. It is supposed to be some of the best over in Japan.