From my understanding, synthetics dont break down or seperate like conventional oil (including slightly contaminated) and this is one of the main reasons to its 15000 or 1 year in engine shelf life. A mechanic I trust told me the biggest reason to run it isnt the viscosity from the intial oil change but its abilty not to break down with contaminates. Hence viscosity stays good through out your planned intervals(with in reason). He did add it will eventually break down but it takes alot of wear and contamination. But we are comparing crude to full science engineered.
I switched to Castrol full syn when it came out with no regrets whats so ever in all my yotas even one with over 200k on the odometer and it loved it. I change my oil every 7500 and it looks like I just changed it. My wifes rig has mobil one and seems like the results are the same( but for me nothing but Castrol
).