But a lot of people here preach that your engine will never know if you use a thicker oil, but the Dodge Hemi tells us otherwise.
I agree with @ARCOgraphite
. I’ve noticed on cars with VVT that oil viscosity can make a difference in throttle response. My Hyundai with the Theta II 2.4 I used 5w-30 for the first few oil changes because that’s what I had on hand. When I used it all up I purchased 5w-20 and I immediately noticed that the throttle was much peppier when accelerating from a stand still. It took me about a week to get used to the throttle response with 5w-20 oil. I never would have believed it until experiencing it myself but I do believe some engines are sensitive to the oil viscosity being used. FWIW, my Theta II uses an electronically controlled intake VVT actuator so maybe only certain systems are more sensitive than others.
The HEMI infers viscosity from oil pressure based on temperature. If that's out of whack it sets a code. This, as I noted, only appears to be an issue if somebody really goes to town deviating from spec (like running 15w-40) or they are in a location where it gets extremely cold.
As to your other point, this is the problem with the "butt dyno".
The difference in visc at operating between a 5w-20 and 5w-30 is minuscule.
Looking at Pennzoil Platinum:
5w-20: 8.46cSt @ 100C, 45.34cSt @ 40C
5w-30: 10.25cSt @ 100C, 55.2cSt @ 40C
At operating temperature you are off by 2.2cSt.
At only 10C cooler, your 5w-20 is 10.56cSt (90C), heavier than the 5w-30 at 100C.
At only 10C warmer, your 5w-30 is 8.44cSt (110C), thinner than the 5w-20 at 100C.
So, a little more love on the pedal to drive up temperature and your 5w-30 is now thinner than your 5w-20. It's a cooler day? Your 5w-20 is now heavier than your 5w-30 was on a hotter day.
Engines and their systems are simply not that sensitive to viscosity, they can't be! Your VVT would completely crap the bed on a cold start if it can't handle a 2cSt difference at operating temperature. At 10C, your 5w-20 is 186cSt, 22x heavier that at operating temperature!
This is the problem with measures driven by emotion and perception rather than actual scientific data. It's very easy to "feel" something that isn't there when you change a variable and expect something. My 6.4L has more tip-in than my 5.7L, not because it's on a heavier oil, but because it makes almost 100HP more, has a significantly different camshaft profile and has completely different programming.
If you have the capability, track your oil temperature. You can calculate operating viscosity from that using the oil data sheet values for what you are running.
For example, -18C, my oil temp in the Jeep was 71C:
This means operating viscosity was 26.69cSt, double what it is at 100C. My VCT and MDS both worked normally, despite that fact.