I've used royal purple oil in our durango and my saturns. I used it in my truck to before I sold it. I had always used amsoil preivous, but they were out one day when i needed oil. I tired the royal purple since that's all they had.
Now, all of them make claims and I've personally noticed nothing when running the amsoil. However, when I switch to royal purple, I noticed my truck idled quieter and started a smoother in the winter. I didn't drive a consistent enough route to notice a difference in MPG's or anything.
When I switched from amsoil to royal purple in my saturn, I noticed the same smoother cold starts and my MPG's went up an honest 1.5 mpg average overall. I write down every time i fill up and do maintenance, so I had records of before an after.