GM’s use 0W-20 now, but I’m not sure about a 0w30.I guess you have a European-spec Mustang? I have read before that Ford recommended different oil in Europe, because they don't have to meet the US fuel efficiency standards there that they do in the US; but this is the first real example I have seen of it.
Simple rule is: use what it says on the cap. Interesting thing is, the cap on my 2010 GT says 5W-20. "They say" this is to get a hair better fuel mileage for US standards; versus the Europe spec oil (per the cap) might protect the engine a little better.
I have never heard of using 0W-30 but I guess that might be beneficial in extreme cold environments. The 0W basically means it thickens less at low temp, than a 5W or 10W. So if you live up near the polar circle somewhere, you might want to take the extra trouble to get 0W-30, but I don't think I have seen it on the shelves around here, so I wouldn't bother.
And yes a good full synthetic is a good idea, even if it isn't really "needed" if you change the oil regularly, per the maintenance minder. I use 5W-20 in the winter (when there are no road track days) and 5W-30 in the summer (because I take my car to track days in the summer).