If I had my engine rebuilt and it leaked oil, I would blame it on the rebuilder and not the oil. Have them fix the leaks and then run whatever oil you want.
I would not change the type of oil to try and cover up a leak on a freshly rebuilt engine.
BTW, I rebuilt my engine last year and although it was my first 911 engine, it does not leak a drop of oil (knock on wood) and I run fully synthetic because of the turbo. If I can rebuild one and have it leak-free anyone should be able to
