I'm not sure - not that synethetic has some sort of super harsh additive package, but I've heard from numerous places (granted, most of them were here on FTE) that many synthetics have excellent detergent properties, and this is part of the problem - not only do they flow better, but they clean away some of the buildup that can block leaks, effectively making the hole bigger. Therefore, it can leak.

I can't vouch for the legitimacy of this claim; I personally haven't seen any concrete evidence that indicates the presence of a higher detergent value, but you are the first person I've seen so far to dispute it.

Now, back to the task at hand - if your oil has been changed regularly, then you really have no need to clean it out. IMO, if it needs to be "cleaned out", then it probably hasn't been taken care of and will need to be replaced anyway. Stick to regular oil change intervals, and forget about the OTC engine flush stuff - it just thrashes your seals.

Take care!

XXL