Your butt dyno must be more finely tuned than mine! I can't tell a difference going from 20 to 40 myself. Also, mpg remains as it always does, between 22-24mpg on my mostly highway drive.I cant read all the posts but I have owned over 30+ panther platform cars and I have safely used 0w-20 up to 10w-30. I highly recommend **NOT** going to a 40w as you can feel it. And you will also loose some MPG. I used german castrol 0w-30 in a 99 CVPI and at first I did not notice why I only got 19 mpg on the flat highway, and admittedly I did not notice the power reduction. When I swtiched back to motorcraft 5w-30 the power and fuel economy was back.
After researching that back in 2006, I found out german castrol of that era was a basically a 40 weight when hot.