The idea that synthetic oil will cause leaks in your engine is totally false, but it's understandable why people think it's true. If you put synthetic oil in an old car with a filthy engine, it will seem like oil is leaking everywhere. In fact, the problem is that the filth was pretty much the only thing that was holding the engine together. The accumulation of particles, along with the irregular molecular structure of regular-old motor oil, keeps the seals and such gummed-up just enough. The streamlined molecular structure of synthetic motor oil does too good a job cleaning out the bad stuff, which is the very stuff keeping the leaks from springing. If you've got a newer car or an older engine with good seals and regular maintenance, synthetic oil can actually help extend your engine's longevity.