Factory fill is currently still 5W-30 for the UK market
You can choose 5W-30 or 0W-30 to top up, it doesn't matter.
You can even mix in non 504 00 spec oil as a top up as long as you don't put more than 0.5 litres in between oil changes.
(see your owners manual page 215 for the full details)
Eventually more manufacturers may move towards 0W-30 as factory fill, as this reduces start up friction in winter, and gives much better mpg figures for selling cars. A bit like the 'defeat device' on the diesels
As for what the dealer is pushing - this is entirely down to marketing and pricing, best deals etc.
But for the UK, there's really no reason to use 0W-30, 5W-30 is better suited, especially as the engine/car gets older.