Scientifically compelling eh? What, was you planning on paying me for my opinion?
I merely shared my experience. There's parasitic drag on the internals though over the decades of driving various vehicles I've never run across as much as I feel with this auto. That leaves the turbo and the actuators as this is the only auto I've owned that came so equipped.
The exhaust CVVT alone, Vs none, on this engine is good for 25HP. The intake CVVT plays an even greater role hence early engines having only intake camshaft VVT.
This style of the YF was used for a few years. Then the hydraulic VV intake was upgraded to electrically operated. This allowed more manipulation as they could now incorporate the intake into the mapping electrically. And perhaps help discourage the use of 5w-20. Which was somewhat en vogue in the early years by individuals and dealerships alike.
I do not know if your 2016 Kia 2.0T has the same hydraulic camshaft actuators.
I know a fellow that drag races. He turns 3 sec flat 60'. He dynoed his engine with his preferred Valvoline 15W-50. Then he drained it replacing with 30. Found a whopping six HP. He drained it and replaced the 30 with 15W-50.
Parasitic drag from heavier oil isn't much.
That leaves the turbo and/or the hydraulic camshaft actuators. One or the other or both.
I choose the later...
Why?
Well, you revived a thread that had been dormant for three months to insert your opinion, so I figured I'd address it with some facts. The point is that the difference between visc on the bottle at temp is VERY small relative to the impact temperature has on it.
Let's use some data points to illustrate here, just for fun.
Two oils, your 5w-40 and a ILSAC 5w-30, to give us the biggest difference, since a Euro 5w-30 will be far closer to our 5w-40 viscosity-wise.
1. Mobil 1 Formula M 5w-40
40C Visc: 79cSt
100C Visc: 13.2cSt
2. Mobil 1 5w-30
40C Visc: 62cSt
100C Visc: 11.1cSt
So, if both oils at are temp, we are looking at a difference of just 2.1cSt, but look at how that grows and how much viscosity increases as the temperature drops.
At just 10C cooler (90C) our 5w-30 is heavier than our 5w-40, having a visc of 13.8cSt. This is why I asked of you tracked oil temperature. Conversely, at just 108C, our 5w-40 is the same visc as our 5w-30 at 100C.
In a vehicle without a coolant/oil heat exchanger, oil temp will be all over the map. Ambient temp can have a significant impact on it, and thus viscosity. Load and RPM also can rapidly change oil temperature. On the other hand, in an engine with a coolant/oil heat exchanger, you are keeping the oil very close to coolant temperature where the viscosity difference is quite small.