Switching to synthetic at high mileage is not recommended due to the "cleaning" properties of the detergents in the oil which can dislodge any built up gunk in minute cracks in seals from mineral oil over time. This is where you get the "dont switch to synthetic or you'll develop leaks"...youre half right.

I run Castrol full syn 5w30, much better than the mobil i ran for years.