Am I right in thinking that because fully synthetic oil is artificially made, it can with stand higher temperatures within the engine (mainly when a turbo is operating) and therefore is less prone to being split into fractions (similar to the principal of how crude oil is separated into fractions in the first place)?
Also, this is the reason "long life" oil is fully synthetic as it can withstand the repeated heat cycling of the engine, where as semi-synthetic will eventually break down and require changing sooner.
I may be wrong... (I love science, me.)