I really think this debate about organic vs. synthetic oil and the potential to seep through seals and gaskets is kind of a stupid academic argument. Any kind of weakness in the engine seals and gaskets is going to become more evident as time goes on, regardless of the molecule size. If you're using a particular brand of oil because you're worried about it finding its way out of seals, you're worried about the wrong thing. More so if you own a better beast like an M3.
I spoke to some experts today about the "switching back" issue, they say that it isn't an issue and that you can do it. By experts, I mean people who race hotrods and are engine machinists (20+ years experience.)