Highway driving is the
easiest on an engine, even if you are going 80. In most temps, say from freezing and above, a 0W30 will usually be thicker than the 0W20. But when it gets to freezing and well below, the cold cranking viscosity on both oils is tested at -35C.
PQIA Cold Crank Viscosity
Of course at operating temp, the 0W-30 will be thicker than the 0W-20, but possibly not by very much. You have thick xW-20's and thin xW-30's and the operational viscosity may or may not be that different. The second number on the bottle represents a viscosity
range.
That said, 0W-20's do very well in these cars if you are stock and are not on a race track.