The leaking occurs when the buildup in the engine is removed, most synthetic oils have better cleaning agents than the cheaper stuff. For t his reason i wouldnot switch to SYnthetic in a motor with 100k+ miles and 8-10 years of street life on it already.
I use mobile 1 in all my vewhicles and change the filter at 3k miles, top off, and go another 3k, this offsets the cost and i have seen private oil tests showing where just by changing the filter in syn oil theat all the parameters stay in check for 12k miles or more. But i dont go that far, it's just my preference.
i tore the motor down from my old mustang when it gor wrecked at 128k miles, started that regiment at 65K when i bought it, there was literally NO deposits in the motor at all, is was super clean even in the most common areas like the oil pan and in the valleys around the valve covers.