My car doesnt have much HP to begin with, but I am almost positive I notice a difference with Amsoils 0w-30 over there 5w-30. I could be wrong though and it might be one of those "my car is clean and feels much faster!"
Thats a pretty close VI to call if there was a marked difference but here is one for you.
My Briggs and Stratton lawn tractor had 10/30 Pennzoil and Synergyn additive in it when breaking in this summer.
Late Summer I wanted to see through analysis what was in the Phillips HDII 15/40 oil and the Schaeffer #132 additve added at 2 oz per quart so I put it in and run it. Same ambient 85F as before,absolutely no carb adjustments and the motor would idle much lower because of the drag with the mix which was a 50wt when analysed.
The thicker VI costs HP and the smaller the motor the more pronounced it is.I think the first oil and additive was a bit more slippery as well which could factor in on some oils in the same wt I am guessing,,saying the more slip = less friction, the less viscous,the less drag