My 1991 911/964 TT (after market) began leaking oil several years ago. It was mostly due to the valve covers. They are the heavy black/ silver, finned type. I replaced the gaskets which was very time consuming due to all of the turbo piping. Prior to reinstalling them, I had them milled at a machine shop. Thr lowers were the worst. Cost for milling was $100/pair. No more leaks, I highly recommend the procedure. VETTEKILLER