For Todays GM cars, Synthetic is fine. Which one you use may vary how long you can leave it in there for. Running a non synthetic in Todays GM cars would pretty much cause you the opposite of leaks and cause all kinds of bad things just short of powertrain warranty issues. The 1.4T in question works extremely well on synthetic oils vs the semi synthetic minimum requirement.Any time I have put synthetic in an older motor its caused leaks so do not want to go there.
In short, the Dexos 1 requirements were in the process of changing to full synthetic IIRC. My old dealer has since changed their semi $39 oil and tire rotation package into a full synthetic $49 package. Not sure when that change happened as I just noticed it beginning of the month.