I've looked up the various cars our engine is used in, and every option from 0W-20 to 10W-40 are recommended in the user manuals.
The fiance's 2018 manual reads 0w-20 so they've probably just switched everything, 2015's say 5w-30, long story short, it's not producing enough power or has close enough tolerancing to really matter, though running a thinner oil in the winter can be a good idea depending on where you live. 5W should be fine down to -25C without causing any additional strain on the pump, but if you're anywhere that gets to -40 you'd want to make use of the block heater, where 0W would just be at its normal limit.
Unless you're abusing the motor, conventional vs synthetic is really just up to preference
View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...