I am guessing the main dealer recommended changing to a thinner oil.
Earlier TwinAir Turbos were recommended SAE 5W-40 ACEA C3, but this changed to SAE 0W-30 ACEA C2 for later TwinAir Turbos.
(It could be argued that C2 is a lower spec than C3, but I would say different not lower).
Whether this change was to improve the engine protection (at the expense of the turbo ?) or because changes were made to the engine which required a thinner oil is unclear.
Early non-turbo TwinAirs were recommended 0W-30, so my view is that this is the correct oil for the engine, but that Fiat initially called up the thicker oil as a compromise for the engine and turbo, then had too many engine failures and shifted the oil selection to better suit the engine.