most people hear that sythetics cause oil leaks because when it was first introduced they didnt swell seals as well as conventional oil. Thats been fixed for the most part with additives though, and people usually are just repeating what they have heard. Kind of like needing to change the oil every 3k miles, that may have been the standard in the 70-80s, but engine tolerances and oil additive technology have made that old rule not true anymore, most new cars can got 7-10k miles without an oil change