You're mistaken. There is no such "rule". It may be that most synthetics are 0w40, but I wouldn't make that claim as I don't know.5w-40 is not synthetic, it would be 0w-40, all 100% synthetic oils are 0-w. As far as I know you can only use full synthetic oil in the Dart.
I have the 1.4 turbo and wondering about what brand oil people are using for the synthetic 5W-40 and also oil filter. Concerning the filter is it screw on or canister?
Larry J - 2013 Aero 1.4 DDCT