For what it's worth, I went from using regular semi-synthetic franken-blends in my car (anywhere from 1/5 synth to maybe 1/2 synth max), to 100% full Pennzoil Ultra Platinum, and I wouldn't say that I've really seen any statistically significant improvement in mileage over the roughly 6300 miles that I've had the PUP in, over what I was getting with the frankenblend.
It's probably there, but in 6,300 miles, I'm not seeing enough of it to be able to distinguish it from the background noise.