Synthetic oil is not what causes the leak. The leak is already there, the synthetic causes it to show its head. Synthetic has an extra additive in it that makes it slipperier, in order to get to more parts of your engine. It does require any flushing to change to it, just do it. There is a semi synthetic oil called High Mileage, or even High performance oil, that may not cause a super ripple in your cars leaking problems.