Before I started using synthetic oils, I spoke to an Amsoil rep at a trade show and he warned me that syn. lubes,due to their smaller molecular structure and lower surface tension,would find places to leak where conventional oils wouldn't.Some of the engines in various trucks and equiptment that were switched to syn. started leaking upon the changeover.
I've found that use of silicone sealants with all gaskets seems to be the answer.I still have a few motors that leak at shaft seals,even after seal replacement.It seems that anything less than a perfect, new gasket or seal will allow synthetics to find a way out.
Even new vehicles,which I change to synthetics after a 10000 mile break-in,
develope aleak or two,mostly inareas where oils puddle against gaskets or seals most of the time,like bottoms of valve covers,oil pans,etc.
It seems that some of the properties that make synthetics work so well also cause them to find their way out of where they belong.
Best answer sofar seems to be to seal up what you can and live with what you can't.
Good Luck,George