Why Don't I Want To Run Synthetic Oil In A Cummings Turbo?

I believe this is a misunderstanding about the use of synthetic oils, Amsoil or any others for that matter. The use of synthetic oils in an engine that leaks will Not "make them bigger." If there's a leak its due to bad seals or some other pre-existing condition they were not aware of synthetic oils will probably "find it" and leak through it. This is one of the physical characteristics of synthetic oils - small regular molecular geometry, very slippery, very good heat stability, etc. But this is also one reason synthetic oils get a bad rap. People blame synthetic oils for 'causing a leak.' 99% of the time the leak was already there but it was occluded by gunk &/or varnish. One of the components of most modern synthetic oils has excellent solvency and will slowly clean out the varnish exposing what otherwise was a 'weak link'. Modern engine oils also contain a bit of seal swelling additive as a matter of course so its possible the leak might stop.
Using high quality synthetic oils in a turbocharged engine is an excellent idea for the above mentioned reasons: excellent heat stability - think about the temperatures turbo units reach and why the unit mfr's recommend a brief cool down prior to shut down and you'll understand why that's important, excellent hi temp-hi shear, excellent oxidation resistance (relates to heat stab.), excellent viscosity index numbers ( how much does it thin down when it gets hot )